AVIATION 


THE  MACMILLAN  COMPANY 

NEW   YORK   •    BOSTON   •    CHICAGO    •   DALLAS 
ATLANTA    •   SAN  FRANCISCO 

MACMILLAN  &  CO.,  LIMITED 

LONDON    •   BOMBAY    •  CALCUTTA 
MELBOURNE 

THE  MACMILLAN  CO.  OF  CANADA.  LTD. 

TORONTO 


AVIATION 


THEORICO-PRACTICAL  TEXT-BOOK 
FOR  STUDENTS 


BY 

BENJAMIN  M.  CARMINA 

ASSISTANT  CHIEF  INSTRUCTOR  AT  THE  Y.  M.  C.  A.  AIRPLANE 
MECHANICS'  SCHOOL,  CHARTER  MEMBER  AND  LEC- 
TURER OF  THE  AERONAUTICAL  SOCIETY 


fork 
THE  MACMILLAN  COMPANY 

1919 

All  rights  reserved 


COPTRIOHT,    1919 

BY  THE  MACMILLAN  COMPANY 
Set  up  and  electrotyped.    Published  June,  1919 


TO 
THE  GENIUS  OF  MAN 

THE  CORRECTOR  OF  NATURE 

THE  CREATOR  OF  WINGS 
THAT  BEAT  BIRD  AND  WIND 


56JWS1 


PREFACE 

In  the  compilation  of  this  book,  the  guiding  principle  has 
been  to  use  matter  of  actual  theorico-practical  value  to  the 
aviation  students  to  enable  them  to  work  knowingly. 

For  a  given  aeroplane  part,  the  most  common  term  has 
been  chosen  out  of  the  maze  of  confusing  terminology, 
which  ought  to  have  been  relegated  into  oblivion  long 
ago  to  facilitate  the  study  of  one  of  the  greatest,  if  not 
the  greatest,  products  of  the  human  mind.  Admittedly, 
the  aeroplane  is  in  its  infancy,  but  an  infant  that  can 
go  at  such  a  high  rate  of  speed  and  perform  such  marvel- 
ous feats  certainly  deserves  more  than  passing  attention, 
and  it  is  high  time  to  standardize  the  names  of  its  parts, 
at  least.  Although  wrong  as  any  other,  the  term  " plane" 
has  been  used  to  designate  a  wing  or  a  wing-like  structure, 
because  it  is  incorporated  in  the  very  word  "aeroplane," 
and  to  have  introduced  a  new  and  proper  term  would  have 
meant  the  changing  of  even  the  name  of  the  machine,  thus 
creating  more  confusion. 

The  appendix  has  been  added  for  the  benefit  of  the  students 
who  wish  to  go  deeper  into  the  science  of  aerodynamics,  and 
to  facilitate  the  task  of  those  who  have  not  the  necessary 
mathematical  knowledge,  the  superficial  elements  of  algebra, 
trigonometry  and  the  metric  system  have  been  given  in  the 
definitions. 

It  is  hoped  that  this  treatise  will  fill  the  long  felt  want  of 
a  theorico-practical  text-book  on  aviation. 

THE  AUTHOR. 


CONTENTS 

CHAPTER  I 

Theory  of  flight.  Planes:  flat  planes,  cambered  planes,  active  and 
passive  drift.  Stability:  longitudinal  stability,  lateral  stability, 
directional  stability,  inherent  stability — longitudinal  dihedral 
angle,  lateral  dihedral  angle,  angle  of  sweepback,  vertical 
stabilizer,  gliding  angle,  propeller  torque 1 

CHAPTER  II 

Aeroplane  construction.  Parts:  uselage,  undercarriage,  center 
section,  wings,  empennage,  wires,  power  plant.  Controls. 
Pontoons.  Materials:  strength  of  materials,  wood,  metal,  fab- 
ric   34 

CHAPTER  III 

Rigging.  Assembling:  fuselage,  undercarriage,  center  section, 
wings,  empennage,  control  wires.  Truing:  fuselage,  under- 
carriage, center  section,  wings — -lateral  dihedral  angle,  angle  of 
incidence,  stagger,  wash  in  and  wash  out,  aileron  droop — em- 
pennage, controls.  Rigging  care  and  faults 70 

CHAPTER  IV 

Propellers:   theory,   problems,   manufacture,   balance,   test,   care, 

boss 83 

CHAPTER  V 

Maintenance:  inspection,  forced  landing,  repairs:  wood,  metal,  sol- 
dering, fabric,  rubber 100 

CHAPTER  VI 

Flight  hints:  methods  of  instruction,  taxying,  elementary  flying, 

stunts 116 

APPENDIX 

Aerodynamical  formulae  and  calculations 133 

DEFINITIONS 
Aviation  glossary,  algebra,  metric  system,  trigonometry 147 

Index 167 

ix 


AVIATION 


AVIATION 

CHAPTER  I 
THEORY  OF  FLIGHT 

PLANES 

Aviation  is  the  branch  of  aeronautics  that  treats  of  the 
gasless  aircraft. 

The  fundamental  law  governing  aviation  is  based  on  the 
resistance  of  the  air  against  a  body  moving  through  it. 

Man's  first  application  of  this  law  to  obtain  flight  is  found 
in  the  use  of  the  kite,  which  is  in  reality  the  forerunner  of 
the  aeroplane, 

In  analyzing  the  process  of  kite  flying,  we  find  that,  in 
order  to  accomplish  flight,  a  natural  current  of  air  must 
blow  against  the  kite  or  the  kite  must  be  dragged  through 
the  air  generating  its  own  artificial  current,  and  that  in 
either  case  the  kite  must  be  at  an  angle  with  the  horizon 
and  not  in  a  vertical  position.  While  it  is  immaterial,  there- 
fore, whether  the  air  attacks  the  kite  or  the  kite  attacks  the 
air,  it  is  imperative  that  the  kite  be  at  an  angle  with  the 
horizon  or  there  will  be  no  flight.  From  experience,  we  know 
this  to  be  so;  now  let  us  see  why. 

Flat  Planes. — If  we  move  through  the  air  a  normal  plane, 
it  simply  pushes  the  air  back  without  accomplishing  any 
work,  because  the  air  meets  the  plane  perpendicularly  and 
slips  off  all  around  the  edges  evenly.  The  air  pushed  back 
by  the  plane  will  exercise  against  it  a  certain  resistance  with 
a  consequent  pressure,  whose  center  will  be  in  the  center  of 
figure.  If  we  double  the  speed  of  the  plane,  the  plane  will 
displace  double  the  amount  of  air  and  the  air  will  strike  the 

1 


AVIATION 


plane  with  double  the  force,  so  that  the  resultant  resistance 
will  be  the  product  of  two  times  the  mass  of  air  engaged  by 
two  times  its  striking  force,  that  is,  four  times  as  great  as 
before  or  equal  to  the  first  amount  of  resistance  multiplied 
by  the  square  of  two.  If  we  treble  the  speed,  the  plane  will 
engage  three  times  the  amount  of  air,  the  air  will  strike  the 
plane  with  three  times  the  force  and  the  result  will  be  nine 
times  greater  or  equal  to  the  first  amount  of  resistance  by 
the  product  of  three  times  three  or  the  square  of  three;  and 
so  forth.  We  can  say,  therefore,  that  the  resistance  of  the 
air  to  a  normal  plane  moving  through  it  is  proportional 
to  the  surface  of  the  plane  and  to  the  square  of  the  velocity. 
Properly  speaking,  for  very  high  speeds  the  resistance  in- 
creases at  a  greater  rate  than  the  square  of  the  velocity, 
until  we  reach  the  5th  power  at  about  800  miles  per  hour, 
when  it  begins  to  diminish  until  it  becomes  less  than  the 
square,  but  for  all  practical  purposes,  we  may  say  that  the 
square  law  holds  good. 

If  we  now  move  through  the  air  an  inclined  plane  A  B 
(Fig.  1),  the  air  strikes  the  under  side  of  the  plane  and  flows 

downward  perpen- 
dicularly. In  so  do- 
ing, while  it  tries  to 
force  the  plane  back- 
ward, in  the  mean- 
time, forces  it  up- 
ward.  In  this  case, 
the  center  of  press- 
ure P  is  toward  the 
front,  because  the 

front  part  of  the  plane  does  most  of  the  work,  as  it  engages 
undisturbed  air.  We  may  consider  this  resultant  force  P  di- 
vided in  its  two  components  L  and  D,  the  first  acting  in 
a  vertical  direction  and  pushing  the  plane  upward,  the  second 
in  a  horizontal  direction  and  pushing  the  plane  backward. 
The  vertical  component  is  the  lift  and  the  horizontal,  the  drift. 


THEORY  OF   FLIGHT  3 

The  angle  a  formed  by  the  plane  A  B  with  the  horizontal 
C  E  is  the  angle  of  incidence.  If  we  lower  the  front  edge  of 
the  plane  still  further  until  it  is  horizontal,  then  the  angle 
of  incidence  will  be  zero;  and  if  we  continue  to  lower  it  still 
more,  the  angle  formed  by  the  plane  below  the  horizontal 
will  be  a  negative  angle  of  incidence. 

The  resistance  of  the  air,  against  an  inclined  plane  moving 
through  it,  is  proportional  to  the  surface,  the  square  of  the 
velocity  and  the  sine  of  the  angle  of  incidence. 

Flat  planes  do  not  give  good  results,  because  the  air  meeting 
the  plane  is  shot  down  vertically  and  the  rear  part  does  little 
work,  as  it  engages  air  which  has  already  a  downward  trend, 
besides  the  fact  that  the  air  rushing  past  the  entering  edge 
of  the  plane  carries  away  part  of  the  air  in  the  rear  of  it, 
causing  a  partial  vacuum,  which  renders  easier  the  work 
of  the  pressure  of  the  air  in  front  of  the  plane  in  pushing  it 
back  and  resulting,  therefore,  in  greater  drift.  In  this  con- 
nection, the  arrangement  of  planes  which  deserves  special 
attention  is  the  tandem  arrangement,  because  it  explains 
the  increased  lift  of  curved  planes. 

If  we  place  three  planes,  equal  in  shape,  dimension  and 
inclination,  one  after  the  other  in  a  straight  line  and  drive 
them  through  the  air,  we  find  that  the  first  plane  lifts  a  great 
deal  more  than  the  second  and  the  third  respectively.  The 
reason  for  this  marked  falling  off  in  the  lift  of  the  rear  planes 
is  that  they  have  to  engage  the  air,  which  was  already  pushed 
downward  by  the  preceding  ones  and  therefore  caused  to 
meet  the  rear  planes  with  a  different  horizontal  velocity 
than  it  met  the  forward  planes. 

It  is  evident  that  if  we  want  to  use  the  tandem  arrange- 
ment, we  have  to  dispose  the  planes  so  that  the  rear  ones 
will  be  able  to  engage  undisturbed  air,  and  to  do  this,  we 
have  to  place  the  second  plane  lower  than  the  first,  and  the 
third  lower  than  the  second;  that  is,  in  steps.  This  differ- 
ence in  level  must  be  proportional  to  the  size  of  the  planes, 
so  that  the  air  moved  by  the  preceding  ones  will  pass  above 


4  AVIATION 

the  rear  planes.  The  best  disposition  will  be  attained  when 
the  sum  of  all  the  spaces  between  the  planes  is  equal  to  the 
whole  area  occupied  by  the  planes.  In  this  way,  we  will  be 
able  to  produce  a  large  lift  per  unit  of  surface  and  a  relatively 
low  drift. 

But  we  can  dispose  the  same  planes  in  another  way, 
which  is  just  as  effective  as  the  step  formation  and  which, 
incidentally,  leads  us  to  the  formation  of  the  curved  planes. 
Cambered  Planes. — As  we  have  just  seen,  an  inclined 
plane,  moving  through  the  air,  leaves  it  at  the  rear  edge 
with  a  downward  motion;  if  we,  therefore,  want  to  use  two 
or  more  planes  one  after  the  other,  we  have  to  place  the 
rear  planes  at  a  greater  angle  than  the  preceding  ones,  so 
as  to  engage  the  air  already  pushed  downward  by  the 
latter. 

Suppose  that  A  B  (Fig.  2)  is  a  plane  inclined  at  an  angle 
of  6°;  if  we  drive  it  forward,  it  is  clear  that  the  air  will  flow 

A  away  at  B  with  a 
downward  trend;  so, 
if  we  want  to  use 
another  plane  C  D 
behind  it,  we  will 
have  to  put  it  at  a 

greater  angle,  say  10°,  and  if  we  want  to  add  a  third  plane 
E  F,  we  have  to  set  it  at  a  greater  angle  than  the  second  plane, 
say  12°.  As  there  is  no  reason  why  we  should  use  three  planes 
instead  of  one,  which  will  answer  the  same  purpose,  we  can 
substitute  for  the  three  planes  A  B,  C  D,  E  F,  the  plane 
A  F,  which  will  have  the  same  shape  formed  by  the  other 
three,  with  the  joints  rounded  off,  so  as  to  present  a  con- 
tinuously curved  stream-lined  surface,  that  is,  a  surface  so 
shaped  as  to  exactly  follow  the  contour  of  the  line  traced 
by  the  successive  positions  of  a  particle  of  fluid  moving  ac- 
cording to  a  determinate  law.  A  stream-line  is  a  continuous 
curve,  as  a  fluid  can  not  instantly  change  its  direction  of 
flow  without  forming  a  detrimental  surface  of  discontinuity, 


THEORY  OF  FLIGHT  5 

as  is  the  case  with  flat  planes.  This  explains  partly  the  reason 
why  curved  surfaces  give  more  lift  and  relatively  less  drift 
than  flat  ones;  and  it  explains  it  only  partly  because  the  planes 
used  to-day  have  a  double  curvature,  one  above  and  one 
below,  differing  in  degree  and  imitating  the  conformation 
of  a  bird's  wing.  Planes  so  shaped  were  at  first  used  in  mere 
imitation  of  nature,  as  in  trying  to  realize  man's  dream  of 
centuries,  the  conquest  of  the  air,  nothing  was  more  natural 
than  to  imitate  the  only  real,  living  flying  machine  in  exist- 
ence, the  bird,  but  the  attempt  failed,  not  because  the 
principle  was  wrong,  but  for  the  great  disparity  for  unit 
weight  between  the  muscular  power  of  man  and  bird  and 
for  the  multiplicity  of  parts  needed,  with  the  consequent 
friction,  which  would  render  uneconomical  even  the  use  of 
motors  to  accomplish  flight  through  such  a  mechanism. 
But  although  the  machine  with  the  flapping  wings,  or  orni- 
thopter,  was  a  failure,  it  played  a  very  important  part  in 
the  solution  of  the  problem  of  aerial  navigation,  for  it  re- 
vealed to  us  the  mysteries  of  the  conformation  of  the  bird's 
wing,  whose  construction  we  imitate  in  the  design  of  the 
successful  flying  machine  of  to-day.  The  revelation  of  this 
natural  secret,  coupled  with  the  knowledge  of  the  laws  that 
govern  the  flight  of  the  kite,  gave  us  the  means  to  conquer 
the  air. 

If  we  move  through  the  air  a  plane  A  B  (Fig.  3),  having 
an  upper  and  lower  camber  as  the  planes  used  to-day,  the 
leading  edge  A  splits  the  air  and  forms  two  currents;  one 
follows  the  lower  camber  and  produces  a  compression,  which 
resolves  itself  in  lift  and  drift  as  in  the  flat  plane,  but,  in 
the  present  case,  it  flows  smoothly  along  the  camber  and 
gives  the  maximum  lift,  although  the  front  part  has  more 
lift  than  the  rear  even  in  a  cambered  plane,  as  it  engages 
always  undisturbed  air;  the  other  current,  striking  the  front 
part  of  the  upper  camber,  glances  upwards  and,  in  rushing 
to  the  rear,  carries  with  it  the  air  lying  between  itself  and  the 
upper  camber,  causing  in  this  way  a  rarefaction  perpendicu- 


6 


AVIATION 


lar  to  the  plane  and  rendering  more  effective  the  pressure 
on  the  lower  camber,  which  tries  to  equalize  the  difference 


Fig.  3 

in  the  density  of  the  air  above  and  below  and  produces  a 
greatly  increased  lift.  The  greater  amount  of  lift  is  due  to 
the  rarefaction  on  the  upper  camber,  which  in  some  planes 
is  as  much  as  80  per  cent,  the  balance,  or  20  per  cent,  being 
given  by  the  pressure  on  the  lower  camber,  while  the  drift 
is  simply  the  horizontal  component  of  this  pressure.  From 
this,  we  see  clearly  why  cambered  planes  are  much  better 
suited  than  flat  ones  to  accomplish  flight. 

A  very  simple  experi- 
ment will  conclusively 
prove  the  lift  due  to  the 
upper  camber. 

If  a  sheet  of  paper  A  B 
(Fig.  4)  is  first  folded,  then 
opened,  without  flatten- 
ing it  out,  and  one  part 
C  is  laid  flat  on  a  board, 
the  other  part  D  forms  a 
curve  behind  the  line  of 
the  fold;  if  we  now  hold 
the  flat  part  and  by  mouth 
direct  a  stream  of  air  parallel  to  it,  the  curved  part  rises 
and,  if  the  current  is  strong,  it  jumps  up. 

In  considering  the  lift  and  drift  of  a  plane,  we  have  to  take 
into  consideration  its  horizontal  and  vertical  projections  or 


Fig.  4 


THEORY  OF   FLIGHT 


equivalents.  The  horizontal  projection  A  C  of  a  plane  A  B 
(Fig.  5)  increases  A  C'  with  a  decrease  in  the  angle  of  inci- 
dence, while  its  vertical  projection  A  D  decreases  A  D', 
and  vice  versa.  The  lift  is  proportional  to  the  horizontal 
equivalent,  the  drift  to 
the  vertical  equiva- 
lent. This  means  that 
the  smaller  the  angle, 
the  greater  the  lift, 
and  the  greater  the 
angle,  the  greater  the 
drift;  but,  on  the  other 


A 


Fig.  5 


hand,  the  increase  of  the  angle  causes  the  plane  to  engage 
more  air,  and  as  in  reality  it  is  the  product  of  the  two  that 
must  count,  that  is,  the  surface  of  the  plane  and  the  mass  of 
air,  an  increase  of  angle  means  an  increase  of  lift  besides 
an  increase  of  drift,  so  that  at  a  certain  angle  the  two  forces 
will  balance.  The  best  proportion  of  lift  to  drift,  or  lift 
drift  ratio,  is  found  for  small  angles,  as,  in  this  case,  the 
proportion  of  the  horizontal  equivalent  to  the  vertical  equiv- 
alent is  the  highest.  In  other  words,  the  nearer  the  plane 
comes  to  the  vertical,  the  greater  the  drift  and,  consequently, 
the  greater  the  power  needed  to  overcome  it,  and  vice  versa; 
which  means  that  the  theory  of  the  plane  set  at  an  angle 
is  the  same  as  the  old  known  theory  of  the  inclined  plane. 

Let  us  make  this  clearer.  If  the  greatest  weight  that  a 
man  can  lift  in  a  perpendicular  line  to  a  height  of  two  feet 
is  150  pounds  and  he  has  to  lift  a  greater  weight,  he  usually 
resorts  to  the  use  of  a  plank,  by  putting  one  end  of  it  on 
the  point  where  he  wants  to  raise  the  weight  and  the  other 
end  on  the  ground,  and  rolling  on  it  the  given  weight  to 
the  given  height.  As  weight  means  gravity,  it  is  clear  that 
in  this  case  the  power  employed  to  lift  the  weight  is  less 
than  that  of  gravity,  and,  consequently,  the  use  of  the  in- 
clined plane  is  very  economical  in  the  expenditure  of  power. 
For  this  very  reason,  the  solution  of  the  problem  of  aerial 


8  AVIATION 

navigation  by  means  of  the  helicopter,  or  machine  intended 
to  fly  by  means  of  horizontal  propellers  which  would  raise  it 
straight  up  from  the  ground,  has  not  been  possible  so  far, 
as  such  a  machine,  to  leave  the  ground,  must  produce  first 
of  all  a  vertical  force  powerful  enough  to  overcome  that  of 
gravity,  and  this  without  considering  the  power  lost  in  con- 
sequence of  the  extreme  fluidity  of  the  air.  Other  consid- 
erations are  against  the  use  of  the  helicopter.  Even  ad- 
mitting that  a  motor  so  light  and  powerful  could  be  found 
to  accomplish  flight  by  such  means,  it  is  necessary  to  use 
at  least  two  propellers,  because  if  only  one  were  used,  the 
propeller  torque,  or  rotary  force  of  the  propeller,  would  cause 
the  machine  to  revolve  in  an  opposite  direction.  Then 
again,  once  the  machine  is  raised  from  the  ground,  another 
propeller  would  be  necessary  to  make  it  move  in  a  horizontal 
direction  or  the  machine  should  be  tilted  so  that  the  same 
propellers  that  raise  it,  cause  it  to  move  horizontally.  And 
finally,  supposing  that  flight  could  be  accomplished  by 
means  of  a  helicopter,  there  is  to  consider  the  ever  present 
possibility  of  the  stoppage  of  the  motor,  in  which  case  the 
machine  would  tumble  down  like  a  plummet.  In  opposition 
to  this  and  in  further  confirmation  of  the  great  superiority 
of  the  machine  using  the  inclined  plane  for  its  sustentation 
in  the  air,  we  will  cite  the  case  of  the  glider  used  in  the  ex- 
perimental stages  of  aerial  navigation,  which  was  sufficient 
to  raise  into  the  air  the  weight  of  a  man  by  means  of  his 
muscular  power;  and  the  glider  was  nothing  but  a  flying 
machine  without  motor  and  propeller. 

In  regard  to  the  shape  of  the  curvature  of  cambered  planes, 
the  best  suited  is  the  parabolic  curve,  with  its  highest  point 
near  the  front  edge.  This  parabola,  as  soon  as  struck  by 
the  air,  pushes  it  downward  with  a  constant  vertical  velocity, 
without  interference  with  the  following  masses  of  air,  in  this 
way  increasing  the  lift  and  decreasing  the  drift. 

As  to  the  degree  of  curvature,  no  definite  rule  can  be 
given,  because  it  must  vary  according  to  the  speed  of  the 


THEORY  OF  FLIGHT  9 

machine;  the  curvature  being  smaller,  the  speedier  the 
machine,  in  order  to  decrease  the  drift.  From  this,  it  follows 
that  the  rule  set  by  the  great,  unlucky  pioneer,  Lilienthal, 
that  the  curvature  be  1/12  of  the  chord  of  the  arc,  can  not 
be  applied  generally.  Most  likely  this  conclusion  was 
reached  through  the  study  of  bird  wings,  but  evidently  we 
could  not  properly  compare  them  with  the  planes  of  a  ma- 
chine, as  the  birds  use  their  wings  both  for  flapping  and  glid- 
ing, while  we  use  the  planes  for  gliding  only.  That  the  same 
curvature  would  be  successful  for  both  cases  is,  therefore, 
out  of  the  question. 

The  angle  of  incidence  of  the  cambered  planes  is  of  the 
greatest  importance;  the  plane  must  be  set  so  that,  in  splitting 
the  air,  it  allows  the  latter  to  flow  above  and  below  without 
disturbing  its  continuity.  What  this  angle  is  to  be,  it  must 
be  arrived  at  according  to  the  shape  of  the  plane. 

From  the  foregoing,  it  is  clear  that  nothing  is  established 
with  certainty  as  yet  in  regard  to  cambered  planes,  and, 
therefore,  we  must  be  guided  by  actual  experience  to  find 
the  best  angle  of  incidence  and  the  center  of  pressure,  which 
differ  with  the  degree  of  curvature. 

Owing  to  the  camber  of  the  planes,  it  would  be  impossible 
to  measure  the  angle  of  incidence,  which  would  vary  at  dif- 
ferent points,  unless  some  means  were  found  to  make  it 
equal  throughout  and  this  is  accomplished  by  using  the 
chord  or  straight  line  A  B  (Fig.  6a)  drawn  from  the  leading 
to  the  trailing  edge,  but,  in  this  case,  when  the  angle  of  in- 
cidence is  zero,  that  is,  when  the  chord  is  parallel  with  the 
horizontal  or  line  of  flight,  the  plane  has  still  lift,  due  to 
the  upper  camber  and,  consequently,  the  real  zero  angle  for 
cambered  planes  must  be  below  that  given  by  the  chord. 
This  point  is  found  by  experiment  in  the  wind  tunnel  by 
lowering  the  leading  edge  D  (Fig.  66)  of  the  plane,  until 
it  is  in  such  a  position  that  there  is  no  lift.  Then,  a  line 
C  E  drawn  from  the  trailing  edge  through  the  width  of  the 
plane,  parallel  to  the  line  of  flight,  will  be  the  neutral  line. 


10 


AVIATION 


This,  therefore,  brings  us  to  the  consideration  of  two  angles 
of  incidence  in  a  cambered  plane ;  one  B  A  F  (Fig.  6a)  formed 

by  the  chord  A  B  with 
the  line  of  flight  A  F, 
or  rigger's  angle  of  in- 
cidence, and  the  other 
G  A  F  formed  by  the 
neutral  line  A  G  with 
the  line  of  flight  A  F, 
or  flying  angle  of  in- 
cidence. 

For     practical    pur- 


Fig.  6 


poses,  only  the  rigger's 


angle  of  incidence  is  used,  as  the  neutral  line  is  imaginary 
and  we  have  no  means  to  find  it  unless  by  experiment  in 
the  wind  tunnel,  while  the  chord  can  always  be  found.  So, 
when  we  say  that  a  plane  is  set  at  a  zero  angle  of  inci- 
dence, we  mean  that  its  chord  is  parallel  with  the  line  of 
flight,  and  if  the  plane  is  said  to  be  at  a  negative  angle  of 
incidence,  it  means  that  the  chord  forms  an  angle  below  the 
line  of  flight. 

The  travel  of  the  center  of  pressure  in  flat  and  cambered 
planes  deserves  our  attention.  If  the  front  edge  of  a  flat 
plane  is  lowered,  the  center  of  pressure  moves  forward  and 
lifts  the  plane,  and  if  the  edge  is  raised,  the  center  of  pressure 
moves  backward  and  lowers  the  plane.  Flat  planes,  there- 
fore, are  stable.  It  is  not  so  with  cambered  planes,  because, 
on  account  of  the  two  cambers,  the  center  of  pressure  is  the 
resultant  of  two  forces  and,  consequently,  it  acts  differently 
than  in  flat  planes;  that  is,  when  the  leading  edge  of  a  cam- 
bered plane  is  lowered,  the  center  of  pressure  moves  back- 
ward, and  if  it  is  raised,  it  moves  forward.  From  this,  it  is 
clear  that  cambered  planes  are  unstable,  because  if  the  lead- 
ing edge  rises,  the  center  of  pressure,  moving  forward,  causes 
it  to  rise  still  more,  and  if  it  is  lowered,  the  center  of  pressure 
moves  backward  and  causes  the  plane  to  lower  still  more; 


THEORY   OF   FLIGHT  11 

but  although  cambered  planes  are  unstable,  they  are  used 
because  they  give  greater  lift  and  smaller  drift  than  flat 
planes. 

Another  factor  of  great  importance  entering  in  the  con- 
sideration of  lift  is  the  proportion  of  the  dimensions  of  the 
plane,  that  is,  the  ratio  of  length  or  span  to  width  or  chord, 
which  constitutes  its  aspect  ratio.  The  greater  the  propor- 
tion of  span  to  chord,  the  greater  the  lift  of  the  plane,  because, 
as  we  know,  the  greatest  part  of  the  work  is.  done  by  the 
front  of  the  plane  and  again  because  all  the  air  entering  at 
the  leading  edge  does  not  flow  to  the  rear,  but  some  of  it  is 
spilled  at  the  lateral  ends  of  the  plane.  The  area  of  a  plane, 
found  by  multiplying  its  dimensions,  is  not,  therefore,  the 
effective  area,  and  for  this  reason,  the  plane  must  be  much 
longer  than  it  is  wide.  From  this,  it  follows  that  the  longer 
the  plane,  the  better  it  would  be  in  respect  to  lift,  but  of 
course  there  is  a  limit.  The  plane  must  be  light  and  strong 
in  the  meantime,  and  we  could  not  build  a  plane  immensely 
long  without  increasing  its  weight  beyond  the  limit  imposed 
by  the  aerodynamical  laws,  for  the  reason  that  the  volume, 
and  therefore  the  weight,  of  a  body  increases  as  the  cube  of 
the  linear  dimensions,  and  the  surface  as  the  square  of  the 
dimensions,  and,  consequently,  the  relations  between  weight 
and  surface  would  be  completely  disturbed.  Therefore,  it 
is  better  to  have  several  superposed  planes  of  reasonable 
dimensions,  say  6  to  1,  rather  than  one  of  great  length.  In 
regard  to  width,  it  is  to  be  observed  that  while  narrow  planes 
give  greater  lift  than  wide  ones,  they  do  not  offer  the  same 
safeguard  in  minimizing  the  fall  of  a  flying  machine  in  case 
of  a  compulsory  glide  from  a  height.  As  we  see,  therefore, 
there  is  a  limit  for  both  dimensions,  span  and  chord. 

If  the  span  of  a  plane  A  (Fig.  7)  is  30  feet  and  the  chord 
is  6  feet,  its  aspect  ratio  is  30:  6  =  5.  If  we  cut  it  in  half 
lengthwise  and  we  put  the  rear  half  B  alongside  the  front 
half,  then  the  aspect  ratio  would  be  60:  3  =  20.  The  former 
would  be  a  low  aspect  ratio  and  the  latter  a  high  aspect 


12 


AVIATION 


ratio.     While  in  both  cases  we  have  the  same  surface,  the 
second  plane  would  be  much  more  efficient  than  the  first, 


-50' 


•30'- 


Fig.  7 

because  in  taking  the  rear  part  of  the  plane,  where  it  was 
doing  very  little  work,  and  putting  it  in  front,  where  it  will 
do  the  greatest  work,  we  have  greatly  increased  the  lift. 
Besides  this,  there  is  to  consider  the  spill  of  air,  which  is 
halved,  in  the  former  plane  taking  place  along  two  edges 
six  feet  long,  and  in  the  second  along  two  edges  only  three 
feet  long.  For  the  same  reason,  if  we  were  to  turn  the  plane  A 
so  as  to  offer  to  the  air  the  smaller  side  (Fig.  8),  the  lift 
would  be  immensely  reduced,  because  of  the  great  spill  of 
air  and  the  small  section  of  the  plane  doing  effective  work 
ir  front.  In  conclusion,  we  may  say  that  a  high  aspect  ratio 
is  the  best,  but,  on  the  other  hand,  it 
would  be  impossible  to  build  a  plane 
with  a  very  high  aspect  ratio,  as  it 
would  be  necessary  to  increase  the 
thickness  of  the  frame  work  used  in  the 
construction  of  planes  to  such  a  point, 
that  what  we  would  gain  in  lift,  we 
would  lose  in  weight.  It  is  for  this 
reason  that  we  resort  to  the  grouping  of 
planes  in  superposed  fashion.  By  ar- 
ranging the  planes  in  this  way,  we  get 
about  as  much  lift  as  with  one  long 


Fig.  8 


plane  equal  to  the  sum  of  their  dimensions,  a  good  saving  in 
weight  and  great  strength  of  construction. 

In  the  arrangement  of  superposed  planes,  care  should  be 
taken  to  make  the  gap  or  interplane  distance  at  least  equal 
to  their  width,  otherwise  there  will  be  interference  and  the 
lift  will  be  diminished.  Interference  is  the  detrimental  effect 


THEORY   OF   FLIGHT 


13 


produced  in  the  gap  by  the  rush  of  air,  or  wash,  which  dis- 
turbs both  the  rarefaction  of  the  top  camber  of  the  lower 
plane  and  the  compression  of  the  lower  camber  of  the  upper 
plane.  The  greater  loss  of  lift  is  in  the  lower  plane,  because 
in  this  case  it  is  the  rarefaction  which  is  disturbed  and  we 
know  that  it  is  from  the  rarefaction  that  we  get  the  greater 


Fig.  9 

amount  of  lift.  For  parity  of  surface,  two  superposed  planes 
give  about  15  per  cent  less  lift  than  one  single  plane.  The 
effect  of  interference  could  be  eliminated  by  spacing  the 
planes  far  apart  one  from  the  other,  but  as  wooden  sticks 
or  struts  are  used  to  accomplish  this,  they  should  be  made 
so  thick  that  their  weight  and  resistance  would  cause  a  loss 
instead  of  a  gain.  The  best  way  to  diminish  interference  as 


14  AVIATION 

far  as  possible,  without  unduly  increasing  the  weight,  is  to 
stagger  the  planes,  that  is,  to  dispose  them  in  steps  (Fig.  9). 
If  the  top  plane  is  forward  of  the  lower  plane,  the  stagger  is 
positive  (Fig.  9a);  if  the  position  is  reversed,  it  is  negative 
(Fig.  96);  and  if  there  is  no  stagger  at  all,  it  is  zero  (Fig.  9c). 

Sometimes  in  superposing  planes,  the  top  one  is  made 
_  longer  (Fig.  10)  as  in 

this  case  the  extension 
gives  the  full  amount  of 

,  lift,  having  no  plane  on 

Fig.  10  ^ne  under  side  to  pro- 

duce interference. 

Active  and  Passive  Drift. — We  have  considered  so  far 
the  best  angle  and  disposition  of  the  planes  without  men- 
tioning either  the  means  to  keep  them  rigidly  in  place  to 
maintain  the  angle  and  disposition  given  or  those  to  furnish 
the  motive  power.  It  is  evident  that  a  framework,  an  engine 
and  a  propeller  are  necessary  to  obtain  our  object.  The 
embodiment  of  these  different  parts  in  a  unit  constitutes  the 
aeroplane,  which  is,  therefore,  a  power  driven  aircraft  sus- 
tained in  flight  by  the  reaction  of  the  air  against  planes  set 
at  an  angle  with  the  line  of  motion.  It  is  distinguished  as 
monoplane  and  multiplane,  according  to  the  number  of 
superposed  planes  used ;  the  biplane  and  triplane  being  simply 
particular  cases  of  the  multiplane. 

The  shape  of  an  aeroplane  very  closely  resembles  that  of 
a  bird,  both  having  a  body,  legs,  wings  and  a  tail,  the  only 
difference  being  that  the  bird  has  movable  wings,  which 
furnish  both  motive  power  and  sustentation,  while  the 
aeroplane  has  rigid  wings  for  sustentation  only  and  the 
power  is  furnished  by  a  motor,  whose  rotary  motion  is  trans- 
formed into  a  linear  motion  by  means  of  a  propeller  attached 
to  it.  As  the  machine  moves  through  the  air,  there  is  resist- 
ance against  the  planes  as  well  as  against  the  framework. 
The  resistance  against  the  planes  is  active,  as  it  gives  lift 
besides  drift,  but  the  resistance  against  the  other  parts  is 


THEORY  OF   FLIGHT 


15 


all  passive  drift  and  must  be  overcome  in  order  to  accomplish 
flight.  To  diminish  this  passive  drift  as  much  as  possible, 
the  different  parts  used  in  a  machine  are  given  a  special 
shape,  which  has  been  found  to  be  the  best  suited  for  the 
purpose. 

If  we  move  through  the  air  a  body  A  B  C  D  (Fig.  11), 
having  right-angled  corners,  the  air,  coming  in  contact  with 


Fig.  11 

the  front  part  of  the  body,  jumps  off  at  the  corners  and  falls 
toward  the  rear,  until  it  meets  again  at  a  certain  point  E. 
Between  this  point  and  the  rear  part  of  the  body  a  vacuum 
is  formed  which  retards  the  forward  motion  of  the  body. 
This  is  due  to  the  fact  that  the  air  meeting  the  corners  can 
not  instantly  change  its  direction  of  flow  and  follow  the 
shape  of  the  body.  If  we  round  the  front  corners,  the  air, 
instead  of  jumping  off,  flows  gradually  along  the  curvature 
and  the  sides,  meeting  at  a  nearer  point  F  in  the  rear.  If  we 
cut  the  sides  tapering  down  to  a  point  toward  the  rear, 
then  the  air  follows  exactly  the  contour  of  the  body,  eliminat- 
ing the  formation  of  the  vacuum.  The  body  will  then  be  so 
shaped  as  to  be  blunt  at  the  front  part  and  thin  at  the  rear. 
This  is  a  stream-lined  body  and  the  ratio  of  length  to  width 
is  its  fineness.  The  fineness  of  a  stream-lined  body  is  pro- 
portional to  the  velocity,  that  is,  the  thinner  the  body  the 
better  suited  to  move  through  the  air  at  a  high  velocity, 
because  the  air  has  less  tendency  to  jump  off  in  meeting  the 
blunt  part.  If  the  body,  instead  of  being  rounded  off  at 
the  front,  had  a  sharp  end,  it  would  split  the  air  more  easily, 


16  AVIATION 

but  this  would  be  a  loss  instead  of  a  gain,  because  when  the 
air  meets  the  blunt  part  and  is  split,  it  forms  a  vacuum  G, 
which,  being  in  the  direction  of  motion,  is  beneficial,  both 
because  it  helps  move  the  body  forward  and  because  of  the 
saving  in  the  weight  of  material  by  cutting  off  the  sharp 
edge.  This  small  vacuum  in  front  of  a  stream-lined  body 
is  known  as  Phillip's  coefficient.  When  a  body  can  not  be 
stream-lined  by  cutting  it  into  shape,  then  additional  parts 
of  wood,  metal  or  fabric  are  used,  so  as  to  give  it  the  proper 
form  for  least  resistance. 

Another  factor  which  increases  the  passive  drift  is  the 
skin  friction.  In  regard  to  this  point,  there  is  a  great  deal 
of  controversy,  some  authorities  saying  that  it  is  due  to  the 
roughness  of  surface;  others,  that  it  is  the  rubbing  of  the  air 
against  the  layer  of  air  which  surrounds  all  bodies  and  ad- 
heres to  them  even  when  in  motion.  While  this  point  is  still 
in  doubt,  what  is  positively  known  is  that  the  coefficient  of 
skin  friction  is  inversely  proportional  to  the  area  and  the 
velocity,  that  is,  the  greater  the  surface  and  the  greater  the 
velocity,  the  smaller  the  amount  of  skin  friction. 

To  still  diminish  the  passive  drift,  advantage  is  taken 
of  the  shielding  offered  by  one  body  on  another  following 
in  its  wake  within  certain  limits.  It  has  been  found  out  by 
experiment  that  if  two  disks  are  placed  one  behind  the  other 
and  a  stream  of  air  is  directed  against  them,  by  moving  the 
rear  disk  away  from  the  front  one  and  noting  the  drift  given, 
at  a  distance  of  1.50  times  the  diameter  of  one  disk,  the  re- 
sistance of  both  is  a  minimum  or  75  per  cent  of  that  of  one 
single  disk;  then  it  increases  to  a  medium  at  2.15  diameters, 
becoming  equal  to  that  of  one  disk;  and  to  a  maximum  at 
10  diameters,  equaling  that  of  two  disks.  Why  the  resistance 
decreases  instead  of  increasing  from  zero  to  1.50  diameters, 
it  is  not  very  clear,  but  it  seems  that  when  the  rear  disk  is 
moved  backward  that  far,  the  eddy  currents  formed  behind 
the  front  disk  have  the  effect  of  pushing  it  forward  with  such 
a  force  as  to  diminish  the  backward  pressure  against  both 


THEORY  OF  FLIGHT  17 

disks;  past  that  point,  the  eddies  have  less  or  no  effect  and 
the  pressure  increases,  as  it  should. 

In  the  practical  application  of  the  case  of  the  disks  to 
that  of  constructional  parts,  we  have  to  take  into  considera- 
tion the  dimensions  of  the  sides  exposed  to  the  direction  of 
motion.  In  other  words,  if  the  cross-sectional  dimensions 
of  two  parts  are  1  inch  by  2  inches,  and  they  are  placed 
with  the  1-inch  side  facing  the  direction  of  motion,  the  dis- 
tance between  them  should  be  less  than  10  inches  or,  if  the 
other  side  is  exposed,  less  than  20  inches  to  obtain  a  decrease 
in  drift. 

Evidently,  it  is  not  always  possible  to  take  full  or  even 
partial  advantage  of  the  shielding  effect,  owing  to  construc- 
tional requirements.  We  can  conclude,  therefore,  that  if 
it  is  possible  to  place  parts  of  the  framework  of  a  machine 
at  distances  nearer  than  10  times  the  dimensions  offered 
to  the  direction  of  motion,  there  will  be  a  reduction  in  passive 
drift,  due  to  the  shielding  effect  of  one  part  on  the  other. 

STABILITY 

Equilibrium  is  the  state  of  balance  produced  by  the  mutual 
counter  action  of  two  or  more  forces.  Equilibrium  is  charac- 
terized by  three  phases:  stable,  unstable  and  indifferent  or 
neutral.  A  body  is  in  a  state  of  stable  equilibrium  when, 
being  disturbed,  it  tends  to  return  to  its  previous  position; 
in  this  state,  the  center  of  gravity  of  the  body  is  in  its  lowest 
possible  place.  A  body  is  in  a  state  of  unstable  equilibrium 
when,  being  disturbed,  it  tends  to  move  away  from  its 
previous  position;  in  this  state,  the  center  of  gravity  of 
the  body  is  in  its  highest  possible  place.  A  body  is  in 
a  state  of  indifferent  or  neutral  equilibrium  when  it  will 
keep  its  balance  independently  of  the  position  it  is  put 
in;  in  this  state,  the  center  of  gravity  of  the  body  is  at  its 
center. 

The  best  form  of  equilibrium  is  the  neutral,  but  as  it  is 


18  AVIATION 

not  always  possible  to  attain  it,  the  next  step  is  to  try  to 
obtain  the  stable  equilibrium. 

From  the  standpoint  of  stability,  the  flying  machine 
differs  from  any  other  form  of  locomotion.  Being  prac- 
tically suspended  in  such  a  light  and  subtle  fluid,  as  the  air, 
the  aeroplane  is  apt  to  move  and  oscillate  in  the  direction 
of  all  its  three  axes:  longitudinal,  lateral  and  vertical;  and 
consequently  its  stability  must  be  considered  in  connection 
with  these  three  phases;  that  is,  the  longitudinal,  the  lateral 
and  the  directional  stability.  The  lateral  stability,  again, 
must  be  considered  in  regard  to  straight  flight  and  circular 
flight. 

Longitudinal  Stability. — To  obtain  longitudinal  stability 
it  is  necessary  to  balance  the  four  forces  which  act  upon  an 
aeroplane  through  their  respective  centers,  that  is,  gravity, 
lift,  resistance  and  thrust.  If  these  forces  were  to  act  always 
through  a  common  point,  it  would  be  very  easy  to  obtain 
and  maintain  equilibrium,  but  this  is  impossible  in  an  aero- 
plane. While,  by  a  suitable  disposition  of  the  various  parts 
of  the  machine,  we  can  fix  the  center  of  gravity,  the  center 
of  resistance  and  the  center  of  thrust,  we  can  not  always 
bring  them  in  line,  owing  to  constructional  requirements, 
nor  always  count  on  the  thrust,  which  varies  with  the  varying 
power  of  the  motor  and  will  be  completely  absent  when  the 
motor  is  stopped  and  gravity  supplies  the  gliding  power; 
nor  can  we  fix  the  center  of  lift,  as  it  changes  its  position 
with  a  change  in  the  angle  of  attack.  The  most  we  can  do, 
therefore,  is  to  balance  these  forces  for  the  normal  angle  of 
incidence  of  the  machine  and  introduce  other  means  to  re- 
store the  equilibrium  when  it  is  disturbed  by  a  change  of 
the  angle  or  the  stoppage  of  the  motor.  This  is  effected  by 
additional  horizontal  and  vertical  planes  placed  at  the  rear 
of  the  main  planes,  which  act  either  automatically  or  are 
controlled  by  the  aviator.  Let  us  suppose  that  the  center 
of  gravity  of  the  machine  A  B  (Fig.  12)  is  at  G,  and  that, 
when  in  motion,  the  air  will  exert  a  center  of  pressure  at  P, 


THEORY  OF   FLIGHT  19 

under  the  main  plane  A  C.  The  force  tends  to  lift  the  plane 
A  C  and  to  upset  it.  To  avoid  this,  the  horizontal  plane 
D  B  is  connected  with  the  rear  part  of  the  machine,  so  that 
the  same  air  pressure  will  act  under  it,  and  on  account  of 


Fig.  12 

its  long  lever  arm  C  B,  will  counterbalance  the  force  P  and 
maintain  the  equilibrium  of  the  machine. 

If,  instead,  the  aeroplane  dives,  due  to  a  stoppage  of  the 
motor  or  other  cause,  the  case  is  reversed;  the  main  plane 
A  C  falls,  while  the  plane  D  B  rises  and  the  air  pressure  acting 
on  the  upper  side  of  the  plane  D  B,  forces  it  down  and  re- 
stores the  equilibrium.  Usually,  this  additional  stationary 
plane  is  set  at  no  angle  of  incidence  and  has  just  enough  lift, 
due  to  the  upper  camber,  to  carry  its  own  weight  and  that 
of  the  tail,  so  that  it  is  very  sensitive  to  any  change  of  in- 
clination. 

To  further  supplement  this  righting  force  or  direct  the 
machine  up  or  down,  a  horizontal  rudder  is  provided,  which 
is  manipulated  by  the  aviator  by  means  of  a  control  lever. 
By  increasing  or  diminishing  the  angle  of  incidence  of  the 
horizontal  rudder,  which  brings  about  a  corresponding  mo- 
tion of  the  main  planes,  the  machine  is  caused  to  rise  or 
descend. 

Lateral  Stability. — To  maintain  lateral  stability  in  straight 
horizontal  flight,  the  best  position  of  the  center  of  gravity 
is  below  the  center  of  pressure.  In  this  way,  if  a  side  gust 
of  wind  strikes  the  aeroplane,  compelling  it  to  tilt,  the  center 
of  gravity  is  displaced,  too,  from  its  normal  position,  which 
it  will  tend  to  regain  and  in  so  doing  will  bring  the  aeroplane 
back  into  equilibrium.  But,  evidently,  this  is  only  possible 
when  the  power  of  the  wind  is  not  so  strong  as  to  upset 


20  AVIATION 

completely  the  resistance  opposed  by  the  center  of  gravity. 
In  the  case  of  a  strong  wind,  the  aeroplane,  struck  sideways, 
would  turn  turtle. 

In  regard  to  the  center  of  gravity  below  the  center  of 
pressure,  it  is  to  be  observed  that  it  must  be  neither  too 
near  to,  nor  too  far  from  it.  In  the  first  case,  the  machine 
would  be  too  sensitive  to  side  motions;  in  the  second  case, 
there  would  be  a  swaying  action  which  would  tend  to  destroy 
rather  than  maintain  equilibrium. 

An  additional  means  for  maintaining  lateral  equilibrium 
is  the  warping  of  the  planes  or  manipulation  of  the  ailerons; 
that  is,  the  lifting  of  the  rear  extremity  of  one  of  the  main 
planes  or  wings  and  the  contemporaneous  lowering  of  the 
rear  extremity  of  the  other,  so  as  to  cause  a  difference  in  the 
angle  of  incidence  of  the  two  wings  and  bring  about  a  twisting 
motion,  in  order  to  regain  the  lost  stability. 

Let  us  examine,  now,  the  last  phase  of  lateral  stability, 
that  is,  circular  flight. 

When  an  aeroplane  moves  in  a  circle,  it  becomes  sub- 
jected to  a  new  force:  the  centrifugal  force,  which  tends  to 
drive  the  machine  away  from  the  center  of  rotation.  As 
this  force  is  directly  proportional  to  the  mass  by  the  square 
of  the  velocity  and  inversely  to  the  radius  of  the  circle,  it  is 
greater,  the  greater  the  speed  and  the  smaller  the  radius  of 
the  circle,  and  as  the  only  side  of  the  machine  that  offers 
resistance  to  the  centrifugal  force  is  the  outer  side,  it  is  di- 
verted from  its  course;  but,  on  the  other  hand,  the  outer 
wing,  describing  a  wider  curve  and  traveling  faster  than  the 
inner  side,  passes  through  more  air  and  generates  a  greater 
pressure,  with  the  result  that  the  wing  rises  and  tends  to 
check  the  skidding  tendency.  This  rising  movement,  though, 
must  be  regulated  by  the  manipulation  of  the  ailerons  or  the 
warping  of  the  wings:  by  depressing  one  aileron  and  raising 
the  other  or  warping  the  wings  in  an  opposite  sense  for  an 
amount  proportional  to  the  centrifugal  force,  the  machine 
is  made  to  take  the  curve  with  a  lean  to  one  side,  just  enough 


THEORY  OF  PLIGHT  21 

to  allow  it  to  bank  itself  properly  and  counterbalance  the 
effect  of  the  new  force  caused  by  the  turning  motion. 

The  tilting  of  the  aeroplane,  and  consequently  of  the  air 
resistance,  brings  about  a  decrease  in  the  lift  and,  therefore, 
the  aeroplane  will  sag.  The  aviator  must  figure  on  this 
sagging  motion  before  he  starts  to  turn,  to  be  sure  to  clear 
anything  that  might  be  below  the  machine. 

This  falling  motion  can  not,  of  course,  be  avoided  by  in- 
creasing the  speed,  because  both  centrifugal  force  and  air 
resistance  are  proportional  to  the  square  of  the  velocity; 
consequently,  the  higher  the  speed,  the  greater  the  centrif- 
ugal force  and  the  bigger  the  degree  of  tilting  necessary  to 
overcome  it,  and,  obviously,  the  greater  the  fall. 

Having  seen  the  effect  of  the  new  force  on  a  machine  in 
general,  let  us  consider,  now,  the  behavior  of  a  machine 
having  the  center  of  gravity  in  a  different  position.  There 
can  be  only  three  cases:  center  of  gravity  on  a  level  with, 
above  or  below  the  center  of  pressure. 

In  the  case  of  the  center  of  gravity  on  a  level  with  the 
center  of  pressure,  if  the  machine  is  tilted  just  right,  it  will 
follow  its  right  course  without  skidding;  but  if  it  is  tilted 
too  far,  it  will  slide  toward  the  center  of  the  circle; 
and  if  tilted  too  little,  it  will  skid  to  the  outer  side  of  the 
curve. 

If  the  center  of  gravity  is  above  the  center  of  pressure, 
the  turning  movement  is  facilitated,  because,  in  tilting  the 
machine,  the  center  of  gravity,  being  high,  will  tend  to  cause 
the  machine  to  fall,  so  to  say,  toward  the  center  of  the  circle ; 
but  this  tendency,  being  counterbalanced  by  the  centrifugal 
force,  will  make  the  machine  go  perfectly  around  without 
skidding.  With  this  position  of  the  center  of  gravity,  turning 
movements  can  be  accomplished  at  a  very  high  speed  and, 
therefore,  this  is  the  best  system  for  circling  around. 

When  the  center  of  gravity  is  below  the  center  of  pressure, 
on  account  of  the  tilting  and  the  consequent  decrease  in 
lift,  the  center  of  gravity  tends  to  restore  the  equilibrium 


22  AVIATION 

of  the  machine,  and  to  bring  the  wings  to  a  horizontal  posi- 
tion again,  which,  of  course,  is  against  the  turning  motion 
requirement,  and  the  machine  tends  to  skid. 

The  conclusion  to  be  derived  from  our  analysis  is  that 
the  best  position  of  the  center  of  gravity  for  straight  hori- 
zontal flight  is  below  the  center  of  pressure,  while  the  best 
position  for  turning  is  above  the  center  of  pressure,  but  as 
the  latter  is  the  worst  of  all  to  maintain  equilibrium  in  straight 
flight,  we  can  say  that  the  best  position  of  the  center  of  grav- 
ity is  below  the  center  of  pressure. 

An  aeronautical  engineer,  while  admitting  that  for  the 
present  this  is  the  best  way,  expresses  the  opinion  that  the 
future  will  see  the  center  of  gravity  above  the  wings,  because 
by  that  time  the  aeroplane  will  have  acquired  such  a  great 
rate  of  speed  as  to  render  it  indifferent  to  atmospheric 
currents.  But,  considering  the  ever  present  menace  of 
hurricanes,  it  is  very  doubtful  that  this  will  be  the  case, 
because,  even  admitting  what  he  says  in  regard  to  speed, 
it  is  always  possible  that  a  slackening  in  the  power  of  the 
motor,  if  not  its  complete  stoppage,  will  cause  it  to  diminish 
and  then  the  aeroplane  will  be  left  at  the  mercy  of  the  wind, 
which  may  force  it  to  pay  an  unpleasant  visit  to  Mother 
Earth. 

Directional  Stability. — As  in  the  case  of  the  longitudinal 
stability,  the  directional  stability  is  obtained  and  controlled 
by  means  of  additional  stationary  and  movable  planes.  At 
the  tail  end  of  the  machine,  there  are  a  stationary  vertical 
stabilizer  and  a  rudder.  If  the  machine  side  slips  or  a  gust 
of  wind  strikes  it  sideways,  the  pressure  of  the  air  will  act 
on  the  vertical  stabilizer,  the  tail  will  swing  around,  cause 
the  nose  of  the  machine  to  turn  toward  the  direction  of  the 
side  slip  or  wind  and  right  its  course. 

The  rudder,  instead,  is  moved  at  the  will  of  the  aviator 
and  caused  to  turn  to  the  right  or  left,  thus  bringing  about 
a  corresponding  motion  in  the  machine  and  changing  its 
direction. 


THEORY  OF  FLIGHT  23 

Inherent  Stability. — Analyzing  the  three  different  stabili- 
ties, we  see  that,  aside  from  the  balancing  of  the  four  forces 
acting  on  an  aeroplane,  they  are  obtained  by  means  of 
additional  planes,  some  of  which  are  fixed  and  act  automati- 
cally, and  some  others  are  movable  and  controlled  by  the 
aviator.  This  means  that,  without  the  controlling  hand  of 
a  skilled  and  alert  pilot,  the  machine  would  lose  its  balance 
at  the  first  adverse  condition.  While,  of  course,  it  is  possible 
to  handle  a  machine  of  this  kind,  on  the  other  hand,  its 
operation  is  very  tiresome  to  the  operator,  who  can  not  fly 
for  more  than  a  few  hours  before  he  needs  a  rest,  and  this 
besides  the  fact  that  the  controls  operate  only  as  long  as 
the  machine  has  forward  speed,  failing  which,  the  aviator 
can  not  use  them  effectively.  What  we  need,  therefore, 
is  a  machine  which  acts  automatically,  approaching  as  much 
as  possible  the  neutral  equilibrium,  and  such  an  inherently 
stable  machine  is  in  existence  to-day.  Provided  there  is 
enough  distance  between  a  machine  of  this  kind  and  the 
ground,  to  allow  the  righting  forces  to  become  operative, 
the  machine  can  be  thrown  into  any  position,  even  upside 
down,  and  it  will  resume  automatically  its  normal  flying 
position.  It  is  even  possible  for  the  aviator  to  fold  his  arms 
and  allow  the  machine  to  fly  itself.  The  only  requirement 
in  these  cases  is  that  the  machine  be  at  a  fair  altitude;  so 
that  we  can  assert,  contrary  to  the  popular  belief,  that  in 
height  there  is  safety.  The  disadvantages  of  an  inherently 
stable  machine  are  a  slight  loss  of  lift  and  sensitiveness  of 
control,  but  they,  of  course,  will  never  outweigh  the  all 
important  factor  of  safety  first,  and  after  all,  in  this,  as  in 
any  other  case,  a  compromise  is  effected,  by  which  the  ma- 
chine is  built  with  a  fair  degree  of  both  inherent  -stability 
and  manual  control. 

Inherent  stability  in  an  aeroplane  is  attained  by  placing 
at  angles  its  different  planes,  and  these  angles  are :  the  longi- 
tudinal dihedral  angle  for  inherent  longitudinal  stability; 
the  lateral  dihedral  angle  for  inherent  lateral  stability  and 


24 


AVIATION 


CL 


the  angle  of  sweepback  for  both  inherent  directional  and 
longitudinal  stability. 

Longitudinal  Dihedral  Angle. — The  longitudinal  dihedral 
an^lc  is  the  angle  a  (Fig.  13a)  formed  by  the  prolongation 

of  the  chord  of  a  wing 
with  that  of  the  hori- 
zontal stabilizer  of  a 
machine,  and  it  is  made 
possible  only  by  the  de- 
calage  or  difference  in 
the  angle  of  incidence 
between  the  wings  and 
the  horizontal  stabilizer. 
The  longitudinal  dihe- 
dral angle  is  given  to  an 
aeroplane  to  maintain  in- 
herent longitudinal  sta- 
bility. 

Suppose  that,  while 
flying  at  its  normal 
angle,  the  machine  sud- 
denly dives  and  assumes 
a  tail-high  position  (Fig. 
136).  In  this  case,  as 
the  momentum  keeps 
the  machine  moving  forward,  the  resistance  of  the  air  acts 
on  the  upper  side  of  the  horizontal  stabilizer  and  sends  the 
tail  down,  bringing  the  machine  back  to  its  normal  posi- 
tion. If,  instead,  the  tail  drops  (Fig.  13c),  the  case  is  re- 
versed; that  is,  the  air  strikes  the  under  side  of  the  hori- 
zontal stabilizer  and  sends  the  tail  up  again. 

The  sensitiveness  of  the  tail  in  restoring  the  longitudinal 
stability  depends  on  the  kind  of  horizontal  stabilizer  used, 
which  makes  the  tail  lifting,  semilifting  or  non-lifting. 

A  lifting  tail  has  for  a  horizontal  stabilizer  a  lifting  plane, 
with  the  usual  upper  convex  camber  and  lower  concave 


Fig.  13 


THEORY  OF  FLIGHT  25 

camber  (Fig.  14a),  set  at  an  angle  of  incidence  smaller  than 
that  of  the  wings.  While  this  has  the  advantage  of  lifting 
part  of  the  weight  of  the  machine,  it  has  the  disadvantage 
of  not  being  sensitive  in  restoring  the  longitudinal  stability, 
because  when  the  angle  of  attack 
of  an  aeroplane  changes  and  brings 
about  a  corresponding  change  in 
the  angle  of  the  horizontal  sta- 
bilizer, although  the  latter  gains  or 
loses  more  incidence  in  proportion, 
being  set  at  a  smaller  angle  than 
the  wings,  it  still  retains  an  angle 
of  incidence,  arid,  consequently, 
has  lift,  which  militates  against 
sensitiveness.  In  other  words,  if 
the  angle  of  incidence  of  the  wings  g 

is  3°  and  that  of  the  horizontal  F. 

stabilizer   2°,   and   the   aeroplane 

dives  until  the  angle  of  attack  of  the  wings  becomes  2°,  the 
angle  of  the  horizontal  stabilizer  becomes  1°;  in  this  way, 
the  wings  have  lost  1/3  of  the  angle,  while  the  horizontal 
stabilizer  has  lost  1/z,  and  the  consequence  is  that  the  tail, 
having  no  more  enough  lift  to  carry  its  own  weight,  falls. 
If  the  case  is  reversed,  that  is,  if  the  tail  drops  until  the 
angle  becomes  3°,  that  of  the  wings  becomes  4°;  in  this  case, 
the  horizontal  stabilizer  has  gained  J/2  of  its  angle,  while 
the  wings  have  gained  1/3,  and,  consequently,  the  tail,  having 
more  lift  than  normally  needed,  rises. 

A  semilifting  tail  has  a  horizontal  stabilizer  with  a  slight 
upper  camber  alone,  the  lower  side  being  flat  (Fig.  146), 
set  at  a  zero  angle  of  incidence.  As  the  lift  is  just  enough 
to  carry  the  weight  of  the  tail  and  the  angle  is  zero,  the 
slightest  diving  motion  of  the  machine  or  dropping  of  the 
tail,  setting  the  horizontal  stabilizer  at  a  negative  or  posi- 
tive angle,  produces  a  quick  righting  force,  which  restores 
the  longitudinal  stability.  This  arrangement  constitutes  a 


26 


AVIATION 


happy  medium  of  lift  and  sensitiveness  and  is  in  general 
use. 

The  non-lifting  tail  has  a  horizontal  stabilizer  with  a 
convex  camber  on  both  sides  (Fig.  14c),  set  at  a  zero  angle 
of  incidence.  As  the  lift  of  one  side  neutralizes  that  of  the 
other,  being  equal  and  opposite,  and  the  angle  is  zero,  this 
kind  of  horizontal  stabilizer  renders  the  tail  the  most  sensi- 
tive of  all,  but  the  wings  must  carry  the  entire  weight  of  the 
machine  and  its  center  of  gravity  must  be  far  forward  to 
balance  the  weight  of  the  tail. 


Fig.  15 

Lateral  Dihedral  Angle. — The  lateral  dihedral  angle  is 
the  angle  a  (Fig.  15a)  formed  by  two  wings  when  they  are 
tipped  upward  and  it  is  given  to  an  aeroplane  to  obtain 
inherent  lateral  stability. 

If  a  gust  of  wind  strikes  the  machine  sideways  and  sends 
one  wing  up  and  consequently  the  other  down  (Fig.  156), 
the  center  of  gravity  G  of  the  machine,  being  displaced,  G', 
tends  to  regain  its  normal  position  and  to  bring  the  machine 
back  to  an  even  keel.  In  the  meantime,  the  wing  that  is  up 


THEORY   OF   FLIGHT 


27 


loses  some  of  its  lift,  due  to  a  smaller  horizontal  equivalent 
A  B  than  before  A  C,  and  tends  to  drop,  while  the  lower 
wing,  having  more  lift  caused  by  a  greater  horizontal  equiv- 
alent A  D  than  before  A  E,  resists  the  dropping  effect  of  the 
higher  wing.  The  outcome  is  that  the  higher  wing,  being 
compelled  to  fall  on  ac- 
count of  both  the  dis- 
placement of  the  center 
of  gravity  and  the  di- 
minution of  lift,  and 
being  in  the  meantime 
resisted  by  the  lower 
wing,  side  slips  toward 
the  direction  of  the 
lower  wing.  This  side 
motion  brings  a  press- 
ure against  the  vertical 
stabilizer  A  (Fig.  15c), 
which  causes  the  nose 
of  the  machine  to  swing 
around  toward  the  di- 
rection of  the  slide;  the 
higher  wing,  being  on 
the  outside  of  the 

curve,  acquires  more  speed  and  climbs  again,  to  fall  back 
again  and  repeat  the  same  oscillating  motion,  with  a  con- 
tinuously diminishing  intensity,  until  the  equilibrium  is 
finally  restored. 

The  swinging  motion  of  the  machine  is  detrimental,  but 
unavoidable,  as  is  also  the  diminution  of  lift,  due  to  the  in- 
clination of  the  wings,  which  have  a  smaller  horizontal  equiv- 
alent than  they  would  have  if  they  had  no  angle,  but  con- 
sidering the  benefit  derived,  it  is  better  to  have  a  lateral 
dihedral  angle,  even  if  some  lift  is  lost. 

The  greater  the  angle,  the  greater  would  be  the  inherent 
lateral  stability,  but  the  smaller  the  lift,  so  that  a  compro- 


28  AVIATION 

mise  is  attained  by  setting  the  wings  at  a  small  angle,  which 
combines  a  fair  degree  of  stability  and  lift. 

Angle  of  Sweepback. — The  angle  of  sweepback  is  the 
angle  a  (Fig.  16a)  formed  by  the  leading  edge  of  a  wing 
with  the  lateral  axis  A  B  of  an  aeroplane.  It  gives  both  in- 
herent directional  and  longitudinal  stability  to  a  machine. 

If  the  aeroplane  is  diverted  from  its  straight  course,  one 
of  its  wings  A  (Fig.  166)  assumes  a  more  inclined  position 
than  the  other  B,  and  the  air,  offering  a  greater  resistance 
against  the  wing  B,  which  presents  an  equivalent  surface 
C  D  greater  than  that  Df  E  of  the  other  A,  forces  the  more 
exposed  wing  B  back  and  restores  the  directional  stability. 

If  the  machine  dives,  the  air  resistance  acts  on  the  upper 
sides  F  and  G  of  the  wing  tips  and  forces  up  the  nose;  if, 
instead,  the  tail  drops,  the  air  acts  on  the  under  side  of  the 
wing  tips  and  forces  up  the  tail,  thus  restoring  the  longitu- 
dinal stability. 

As  the  directional  stability  of  a  machine  does  not  present 
much  difficulty,  besides  the  fact  that  the  vertical  stabilizer 
promotes  it,  and  the  longitudinal  stability  can  be  maintained 
by  means  of  the  longitudinal  dihedral  angle  with  less  loss 
of  lift  than  the  sweepback  entails,  and  also  on  account  of 
difficulty  of  construction,  the  sweepback  is  not  much  used. 

Vertical  Stabilizer. — The  inherent  directional  stability  is 
maintained  in  almost  all  aeroplanes  by  means  of  the  vertical 
stabilizer,  which  is  a  flat  triangular  plane  bolted  on  the  upper 
part  of  the  tail. 

If  during  flight  a  gust  of  wind  strikes  an  aeroplane  on 
one  side  A  (Fig.  17a),  the  pressure,  although  acting  on  the 
entire  aeroplane,  produces  more  effect  on  the  vertical  stabili- 
zer B  on  account  of  its  long  lever  arm  and  causes  it  to  swing 
to  the  opposite  side  C  (Fig.  176).  As  momentum  tends  to 
keep  the  aeroplane  in  its  previous  direction,  the  pressure  of 
the  air  acts  now  on  the  other  side  of  the  vertical  stabilizer 
and  forces  it  back  to  its  former  position,  thus  righting  the 
course  of  the  aeroplane. 


THEORY  OF   FLIGHT  29 

If  the  case  is  reversed,  the  same  principle  holds  good. 
Gliding  Angle. — One  of  the  most  important  points   to 
consider,  in  connection  with  the  inherent  stability  of  an 


<r 

i 


Fig.  17 

aeroplane,  is  that  when  the  motor  is  stopped  and  gravity 
furnishes  the  motive  power. 

It  is  clear  that  in  this  case  the  machine  can  do  nothing 
but  glide  down,  and  to  do  so  safely,  it  is  imperative  that  it 
assume  the  proper  gliding  angle  automatically.  To  this 
end,  the  four  forces  which  act  on  an  aeroplane  are  disposed 
so  that  the  center  of  gravity  G  (Fig.  18)  is  a  little  in  advance 
of  the  center  of  lift  L,  and  the  center  of  resistance  R  a  little 
above  the  center  of  thrust  T.  Due  to  the  disposition  of  the 
center  of  gravity  ahead  of  the  center  of  lift,  the  aeroplane 
would  come  down  nose  foremost,  if  no  other  force  counter- 


30 


AVIATION 


acted  that  of  gravity;  but  when  the  motor  is  working,  this 
opposing  force  is  furnished  by  the  thrust  of  the  propeller, 


Fig.  18 

and  when  it  is  stopped,  the  necessary  force  is  supplied  by 
the  pressure  of  the  air  against  the  center  of  resistance,  which, 
being  above  that  of  the  thrust  and  far  in  advance  of  the 
center  of  gravity,  resists  the  nose-diving  tendency  of  the 
aeroplane  to  such  an  extent  as  to  make  it  assume  the  proper 
gliding  angle  automatically. 

The  location  of  the  point  of  application  of  each  force,  which 
determines  the  degree  of  the  gliding  angle  ABC  (Fig.  19) 

of  an  aeroplane,  determines 

• — ^  also,   as  a  consequence,  its 

T~  • —  _  #   radius  of  action  C  B  or  hori- 


A 

/M 


r 


Fig.  19 


zontal     equivalent    of    the 
gliding  path  A  B. 
The  radius  of  glide  is  altered  by  the  power  of  the  wind, 
being  increased  or  decreased  according  to  the  direction  of 
the  aeroplane  in  relation  to  that  of  the  wind. 

The  gliding  angle  is  usually  expressed  in  terms  of  the 
ratio  of  the  height  of  glide  A  C  to  the  radius  of  glide  C  B; 


THEORY  OF  FLIGHT 


31 


that  is,  if  the  height  from  which  an  aeroplane  starts  to  glide 
is  1  mile  and  the  distance  traveled  in  a  straight  horizonal  line 
in  reference  to  the  ground  is  6  miles,  it  is  said  that  the  gliding 
angle  is  1  in  6. 

As  the  radius  of  glide  is  directly  proportional  to  the  height 
of  glide,  we  have  here  again  the  confirmation  of  the  fact 
that  height  means  safety,  because  it  gives  a  greater  radius 
of  action  and,  therefore,  affords  the  pilot  a  better  opportu- 
nity to  choose  a  suitable  landing  place. 

Propeller  Torque. — Among  the  different  causes  which 
affect  the  lateral  stability  of  an  aeroplane,  one  deserves 
special  consideration :  the 
propeller  torque.  This  ro- 
tary force  tends  to  produce 
an  opposite  rotary  motion 
to  the  point  of  application, 
which,  if  free  to  move,  will 
actually  revolve,  as  is  the 
case  with  the  helicopter  us- 
ing only  one  propeller.  If 
the  propellers  are  two  or 
any  even  number,  the  effect 
of  the  torque  is  eliminated 
by  making  one-half  of  them 
revolve  in  the  opposite  di- 
rection to  the  other  half. 


\A 


Fig.  20 


In  the  case  of  a  machine  having  one  propeller,  the  result 
is  that  one  wing  A  (Fig.  20a)  is  forced  down,  and,  conse- 
quently, the  other  one,  B,  up.  To  correct  this  direct  effect 
of  the  propeller  torque,  the  angle  of  incidence  near  the  tip 
of  the  lower  wing  is  increased  or  washed  in  to  a  degree  neces- 
sary to  give  it  enough  additional  lift  to  bring  the  wing  back 
to  its  normal  position  and  restore  the  lateral  stability.  The 
same  correction  can  be  made  by  decreasing  or  washing  out 
the  angle  near  the  tip  of  the  high  wing  B  (Fig.  206)  to  bring 
it  down  to  its  proper  level.  A  better  system,  though,  is  to 


32  AVIATION 

increase  the  angle  on  one  side  A  (Fig.  20c)  and  decrease  it 
on  the  other  B  by  one-half  the  amount  needed  for  the  total 
increase  or  decrease.  One  reason  why  this  method  is  to  be 
preferred  is  that  an  increase  in  lift  means  also  an  increase  in 
drift,  which  causes  the  wing  with  the  bigger  angle  to  retard 
its  forward  motion  and  to  bring  about  a  consequent  turning 
movement  in  the  machine,  which  will  be  greater,  the  greater 
the  drift,  and  this,  of  course,  is  the  case  when  the  angle  is 
increased  or  decreased  all  on  one  side.  Another  reason  is 
that  when  the  ailerons  are  used  to  restore  the  lateral  stability 
by  bringing  one  up  and  the  other  down,  they  do  not  work 
in  air  with  the  same  density,  because  the  lower  aileron  re- 
ceives the  compressed  air  from  the  lower  camber,  while  the 
upper  receives  the  rarefied  air  from  the  upper  camber,  and 
the  consequence  is  that  the  lower  aileron  is  more  effective 
and  introduces  in  the  meantime  more  drift  than  the  upper, 
with  the  result  that  the  machine  swings  around  towards  the 
side  of  the  lower  aileron,  and  this  makes  necessary  the  opera- 
tion of  the  rudder  in  conjunction  with  the  ailerons  to  keep 
the  straight  course  of  the  machine.  The  smaller  the  angle 
near  the  wing  tips,  the  smaller  the  drift  of  the  lower  aileron, 
the  smaller  the  turning  tendency  of  the  machine  and  the 
smaller  the  angle  of  the  rudder.  By  dividing  the  angle 
equally  between  the  wing  tips,  therefore,  the  amount  of 
drift  is  reduced  to  the  minimum  and  so  the  consequent 
turning  motion,  both  in  regard  to  that  caused  by  the  correc- 
tion of  the  direct  effect  of  the  propeller  torque  and  the  other 
brought  about  by  the  manipulation  of  the  ailerons. 

To  correct  the  indirect  effect  of  the  propeller  torque, 
either  the  rudder  or  the  vertical  stabilizer  is  set  at  an  angle 
toward  the  wing  tip  which  has  less  incidence,  thus  introducing 
an  equal  amount  of  drift  to  that  side  of  the  machine  and 
restoring  the  directional  stability.  As  the  torque  of  the 
propeller  varies  with  the  power  of  the  motor,  being  com- 
pletely absent  when  the  motor  is  stopped,  and  is  not  there- 
fore a  fixed  quantity,  while  the  angle  at  the  wing  tip  is,  it 


THEORY  OF   FLIGHT  33 

becomes  necessary  to  correct  by  manipulation  the  varia- 
tions in  the  stability  of  the  machine,  produced  by  the  change 
in  the  torque.  For  this  reason,  it  is  better  to  use  the  rudder 
in  correcting  the  indirect  effect  of  the  propeller  torque, 
because  the  rudder  can  always  be  moved  at  will  by  the 
aviator,  while  the  vertical  stabilizer  is  bolted  in  place  and. 
once  set,  is  set  to  stay. 


CHAPTER  II 
AEROPLANE  CONSTRUCTION 

PARTS 

The  main  parts  of  a  monoplane  are  four;  those  of  a  bi- 
plane may  be  four  or  five.  We  will  consider  the  case  of  the 
biplane  with  five  parts,  which,  when  named  in  their  assem- 
bling order,  are  the  following:  fuselage,  undercarriage,  center 
section,  wings  and  empennage. 

All  these  parts  are  light  and  strong  structures  produced 
by  a  skillful  combination  of  wood,,  metal  and  fabric. 

Fuselage. — The  fuselage  (Fig.  21)  is  the  main  body  of  the 
aeroplane,  all  other  parts  being  attached  to  it. 

The  wooden  parts  of  the  fuselage  are:  the  longerons,  top  A 
(Fig.  21a)  and  bottom  B  (Fig.  216);  the  struts,  top  C  (Fig. 
21a),  bottom  D  (Fig.  216)  and  side  E  (Fig.  21c);  the  tail  post 
F  (Fig.  21c)  and  the  engine  rails  G  (Fig.  21a).  The  metallic 
parts  are:  the  fittings  H  (Fig.  21c)  with  their  rivets  or  clevis 
pins  and  cotter  pins  or  bolts,  nuts  and  cotter  pins;  the  turn- 
buckles  M  (Fig.  21a) ;  the  cross  bracing  wires,  top  I  (Fig.  21a), 
bottom  J  (Fig.  216),  side  K  (Fig.  21c)  and  internal  L  (Fig. 
21d);  the  nose  plate  N  (Fig.  21o);  the  engine  rails  support 
0  (Fig.  21a);  and  the  reenforcing  struts  P  and  Q  (Fig.  21c). 

At  the  tail  end  of  the  fuselage  is  the  rudder  post  R  (Fig. 
21c)  and  on  the  under  side  is  the  tail  skid  S  (Fig.  21c),  which 
sometimes  is  attached  to  the  rudder  post  and  sometimes 
to  an  independent  piece  T  (Fig.  21c). 

The  longerons  are  made  of  strong  wood,  as  they  must 
stand  all  kinds  of  stresses  without  breaking,  because  to 
change  a  damaged  longeron  means  to  dismantle  the  fuselage 
and,  consequently,  the  entire  machine. 

34 


AEROPLANE   CONSTRUCTION 


35 


The  struts  are  lighter  and  weaker  wood,  intended  to  take 
only  a  compression  stress. 

The  tail  post  is  not  used  in  all  machines,  as  sometimes 
the  rudder  post  has  the  double  function  of  tail  post  and 
rudder  post. 


Fig.  21 

The  engine  rails  are  very  strong  wood,  usually  laminated, 
and  they  are  fixed  in  place  in  the  strongest  possible  way, 
because  on  them  is  bolted  the  motor. 

The  fittings  are  metallic  fixtures  which  connect  the  joints 
of  the  struts  and  longerons.  The  wires  are  also  attached 


36  AVIATION 

to  them  by  means  of  rivets,  pins  or  bolts.  The  fittings  and 
their  fixtures  are  used  in  all  the  other  parts  of  the  machine 
and  they  will  be  omitted  hereafter,  unless  meant  for  a  special 
purpose. 

The  turnbuckles  are  couplings,  with  a  barrel  and  a  right 
and  a  left  hand  eye  screw  or  shank,  used  to  regulate  the 
length  and  tension  of  wires.  The  right  hand  screw  shank, 
which  sometimes  is  split  or  forked,  is  generally  attached 
to  a  fitting.  This  is  done  to  determine  the  turning  direction 
of  the  barrel  in  tightening  or  loosening  a  wire,  as,  in  this 
case,  the  operation  is  that  of  an  ordinary  right  hand  screw 
nut.  The  come  and  go  is  the  distance  the  shanks  can -be 
screwed  in  or  out.  The  turnbuckles  also  are  used  for  all  the 
wires  and  they  will  be  omitted  in  the  descriptions  of  the 
other  parts  of  the  machine. 

The  cross  bracing  wires  are  of  the  greatest  importance, 
as  from  them  depends  the  rigidity  and  consequent  strength 
of  the  entire  machine. 

The  nose  plate,  engine  rails  support  and  the  reenforcing 
struts  are  used  to  give  the  strength  and  rigidity  required 
for  the  installations  of  the  motor,  whose  vibrations  might 
cause  the  loosening  of  some  weak  part  and  cause  a  disaster. 

The  rudder  post  is  a  metallic  tube  to  which  is  hinged  the 
rudder. 

The  tail  skid  is  a  strong  piece  of  wood  and  is  attached  under 
the  tail  of  a  machine  to  carry  the  weight  of  its  rear  portion 
while  on  the  ground  and  to  act  as  a  shock  absorber  and  brake 
in  landing.  It  is  better  to  have  the  tail  skid  attached  to  an 
independent  piece,  rather  than  to  the  rudder*  post,  because 
in  case  of  a  bad  landing,  the  latter  may  be  distorted  so  as  to 
jam  the  rudder. 

The  main  sections  of  the  fuselage  are :  the  engine  section  A 
(Fig.  22),  the  cockpit  B  and  the  tail  section  C. 

The  fuselage  is  covered  all  around  with  cowling  or  fairing 
to  give  it  a  stream-lined  shape.  This  cowling  is  metal  for 
the  entire  engine  section  and  the  upper  part  of  the  cockpit, 


AEROPLANE   CONSTRUCTION 


37 


the  balance  being  fabric,  sometimes  reenforced  by  a  light 
framework  of  wood,  and  this  is  especially  the  case  with  the 
part  that  covers  the  tail  section.  The  cowling  of  the  top 
takes  special  names,  according  to  the  section  it  covers:  hood 


Fig.  22 

D  for  the  engine  section,  cowl  E  for  the  cockpit  and  turtle- 
back  F  for  the  tail  section. 

The  fuselage  is  constructed  in  the  strongest  possible  man- 
ner to  withstand  all  kinds  of  stresses,  and  as  the  other  main 
parts  are  attached  to  it,  its  fittings  are  so  made  that  while 
they  hold  its  own  members  together,  they  are  ready  to  re- 
ceive the  other  parts  of  the  machine. 

The  fuselage  is  usually  made  in  a  square  section,  built 
box  girder  fashion  and  has  a  fineness  of  7.  Although  stream- 
lined to  diminish  the  passive  drift  as  much  as  possible,  this 
shape  does  not  represent  the  last  word  in  science,  it  being 


Fig.  23 

well  known  that  other  experimental  models  have  given  much 
better  results.  The  future  will  undoubtedly  see  the  shape 
of  the  fuselage  so  altered  as  to  give  the  least  amount  of  drift. 
There  is  a  cigar-shaped  fuselage  (Fig.  23)  specially  con- 
structed and  covered  all  around  with  plywood,  so  as  to  form 
one  single  shell  and  for  this  reason  is  called  monocoque, 
which  in  French  means  one  shell. 


38 


AVIATION 


Sometimes  the  common  box  girder  type  of  fuselage  is 
covered  also  with  plywood,  monocoque  fashion. 

There  is  another  kind  of  short  body  or  cut  off  fuselage 
called  nacelle  (Fig.  24),  which  is 
used  for  machines  that  have  the 
propeller  in  the  rear. 
f"~  \¥\       Undercarriage. — The  undercar- 

\  \4    riage  (Fig.  25)  is  that  part  of  the 

\w  ^^^   '     machine   designed   to  support   it 

when  at  rest,  to  absorb  the  shock 
of  landing  and  to  give  clearance  to 
the  propeller  and  wings. 


Fig.  24 


The  wooden  parts  of  the  undercarriage  are:  the  struts  A 
(Fig.  25a)  and  the  spreader  B;  the  metallic  parts:  the  cross 
bracing  wires  C,  the  axle  D,  the  wheels  E,  the  shock  absorber 
fittings  F  (Fig.  256)  and  the  radius  rods  G. 

Besides  these,  there  are  additional  parts  of  rubber,  that 
is,  the  tires  for  the  wheels  and  the  cables  of  rubber  used  as 
shock  absorbers. 

The  struts  of  the  undercarriage  are  made  purposely  weaker 


Fig.  25 

than  the  longerons  of  the  fuselage,  so  that  in  case  of  a  hard 
landing,  it  will  be  the  struts  which  will  break,  as  they  are 
easily  replaced. 


AEROPLANE  CONSTRUCTION  39 

The  spreader  keeps  the  struts  at  the  proper  distance  and 
is  stream-lined  to  diminish  the  drift. 

The  wheels  used  for  the  undercarriage  are  the  same  as 
those  for  automobiles  and  they  run  up  to  32  inches  in  di- 
ameter. The  heavier  the  machine,  the  larger  the  wheels, 
but  the  standard  size  is  26  x  4,  that  is,  a  wheel  of  26  inches 
and  a  tire  of  4  inches  diameter.  The  larger  the  diameter  of 
the  wheels  and  tires,  the  better  suited  to  run  on  rough  ground, 
but  of  course  the  weight  compels  the  limitation  of  the  size. 
The  hubs  of  the  wheels  have  no  ball  or  roller  bearings  to 
be  lighter,  but  the  spokes  are  very  substantial  and  well 
spread  out  to  resist  side  stresses. 

The  tires  are  of  the  double  tube  type,  that  is,  they  have 
an  air  casing  and  a  shoe  or  outer  casing.  The  center  of  the 
shoe  is  harder,  because  it  is  the  part  that  comes  in  direct 
contact  with  the  ground  and  is  called  the  thread.  The  press- 
ure that  a  tire  can  stand  is  20  pounds  per  sectional  linear 
inch,  but  this  pressure  is  never  given,  because  the  tire,  besides 
being  used  to  facilitate  the  run  on  the  ground,  must  absorb 
some  of  the  shock  of  landing.  The  spokes  of  the  wheels 
are  covered  on  both  sides  with  disks  of  metal,  canvas  or  cel- 
luloid to  stream-line  them,  and  in  this  case  they  are  called 
disk  wheels. 

The  shock  absorbers  used  to-day  are  generally  of  rubber, 
in  the  form  of  a  cable  of  strands  of  rubber  covered  with 
fabric,  wound  around  the  undercarriage  fittings  and  the 
shock  absorber  fittings.  It  is  important  that  the  cables  of 
both  wheels  have  the  same  tension,  to  prevent  a  side  motion 
of  the  machine  in  moving  along  the  ground  and  particularly 
in  landing,  when  it  would  be  very  dangerous  and  might 
cause  the  machine  to  turn  over  sideways.  The  strands  of 
rubber  run  from  50  to  300  and  the  size  of  the  cable  from  half 
an  inch  to  one  inch.  The  length  of  cable  used  is  proportional 
to  its  diameter  and  the  weight  of  the  machine.  These  shock 
absorbers  are  preferred  because  they  are  readily  adjusted  and 
replaced,  their  deterioration  is  easily  detected  and  they  ab- 


40  AVIATION 

sorb  much  more  than  steel  per  unit  weight.  They  are  not 
ideal,  though,  because  the  more  they  stretch,  the  less  they  ab- 
sorb, and  a  shock  absorber  should  really  absorb  the  shock  of 
landing  without  giving  a  rebound.  This  could  be  obtained 
by  the  use  of  the  hydraulic  or  oleo  pneumatic  shock  absorbers. 
They  consist  of  two  tubes,  one  inside  the  other,  separated 
by  either  water  or  oil;  the  outer  tube  is  attached  to  the  axle 
of  the  wheels  and  the  inner  tube  to  the  undercarriage.  When 
the  machine  is  in  flight,  the  weight  of  the  wheels  and  axle 
pulls  the  outer  cylinder  down  and  the  liquid  flows  all  in 
the  outer  tube.  When  the  machine  lands,  the  inner  tube 
presses  the  liquid  and  forces  it  inside  of  the  inner  tube  through 
a  valve  at  its  bottom,  and  as  it  goes  in,  the  air  in  the  inner 
cylinder  resists  it,  thus  forming  a  cushion  of  air  which  ab- 
sorbs the  shock,  while  the  liquid,  through  ports  in  the  inner 
cylinder,  is  forced  out  and  back  again  into  the  outer  tube. 
As,  after  landing,  the  shock-absorbing  power  does  not  exist 
any  more,  and  it  is  necessary  to  have  some  of  it  when  the 
machine  is  pulled  about  on  the  ground,  an  additional  spiral 
spring  is  provided,  which  comes  into  play  when  the  inner 
cylinder  reaches  the  limit  of  its  downward  run.  The  liquid 
is  generally  oil,  because  it  is  less  liable  to  freeze  than  water. 
Although  these  are  real  shock  absorbers,  they  are  not  used 
much  on  account  of  their  weight,  complication  of  parts  and 
high  cost. 

A  kind  of  shock  absorber,  which  is  a  combination  of  shock 
absorber  and  wheel,  is  the  Ackerman  wheel,  whose  spokes  are 
S  shaped  for  the  purpose  of  absorbing  the  shock  of  landing. 

An  important  point  to  be  observed  in  regard  to  the  use 
of  the  rubber  shock  absorbers  and  the  Ackerman  wheel 
is  that  in  the  first  case  the  rubber  cable  is  wound  around 
the  undercarriage  fittings  and  the  shock-absorber  fittings, 
which  means  that  when  the  machine  lands,  the  axle  must 
give  and  stretch  the  rubber;  while  the  axle  of  the  Ackerman 
wheels  must  be  rigidly  attached  to  the  undercarriage,  as 
in  this  case  it  is  the  spokes  that  absorb  the  shock. 


AEROPLANE  CONSTRUCTION  41 

The  radius  rods  are  guides  pivoted  to  the  axle  and  the 
struts,  to  prevent  the  possibility  of  the  axle  striking  and 
breaking  the  struts  when  the  machine  lands  and  the  wheels 
and  axle  jump  up. 

Some  machines  have  neither  radius  rods  nor  separate 
shock-absorber  fittings,  in  which  case  there  is  a  special  axle 
fitting,  which  is  a  combination  of  all  these  parts  in  one. 

The  undercarriage  deserves  a  good  amount  of  thought  to 
avoid  damage  both  to  the  aviator  and  the  machine  even 
before  they  leave  the  ground.  The  weight  of  the  machine 
rests  on  the  undercarriage,  therefore  it  must  be  strong  enough 
to  withstand  the  stress  of  its  load  when  at  rest,  more  so  when 
moving,  because  of  the  increased  stress  imposed  by  the 
shocks  imparted  by  the  runs  on  an  uneven  ground,  and  in 
the  greatest  degree  when  landing. 

The  undercarriage  should  be  so  built  as  to  give  a  quick 
start  and  a  quick  stop.  Evidently  these  are  opposite  re- 
quirements, because  if  a  machine  must  start  quickly,  it 
means  that  the  wheels  must  give  the  least  amount  of  friction 
and,  for  this  very  reason,  the  machine  can  not  stop  quickly. 
To  accomplish  both  results,  it  is  necessary  to  have  a  brake, 
which  will  stop  the  machine  at  the  proper  time  in  landing. 
This  mechanical  brake  would  be  a  very  good  addition  to  an 
undercarriage,  because  often  it  happens  that  a  machine  lands 
on  slanting  ground  and  the  aviator  has  no  means  of  stopping 
it  to  avoid  a  smashup;  but,  on  the  other  hand,  the  brake 
requires  a  proper  adjustment  to  avoid  the  tendency  of  the 
machine  to  come  down  on  its  nose  when  the  brake  is  applied. 

The  undercarriage  should  be  well  sprung  and  strong  enough 
to  withstand  rolling  and  side  shocks  without  deflection  or 
fracture,  and  it  should  also  offer  the  lowest  drift  when  in 
flight,  which  means  that  all  its  parts  must  be  stream-lined 
and  so  disposed  as  to  take  full  advantage  of  the  shielding 
effect. 

The  height  of  the  undercarriage  depends  upon  the  diameter 
of  the  propeller,  which  should  have  a  clearance  of  from  one 


42  AVIATION 

to  two  feet  to  prevent  breakage  due  to  the  tilting  of  the 
machine  or  to  the  sinking  of  the  wheels  into  soft  ground. 

The  center  of  gravity  of  the  machine  is  usually  very  near 
the  center  of  the  wheels,  and  therefore  the  tail  section  of  the 
fuselage  and  the  skid  are  built  lightly.  If  the  center  of  grav- 
ity is  too  far  back,  it  -is  necessary  to  have  a  heavy  fuselage 
and  skid,  which,  in  landing,  cause  a  heavy  drop  of  the  tail 
and  a  consequent  increase  of  the  angle  of  incidence  of  the 
wings,  and  the  result  is  that  the  machine  rebounds.  A  heavy 
fuselage  means  greater  frictional  resistance  for  the  skid,  which 
prevents  a  rapid  start. 

If  the  machine  has  a  lifting  tail,  the  tail  end  of  the  fuselage 
and  the  tail  skid  can  be  built  much  lighter  than  when  the 
tail  is  non-lifting. 

Center  Section. — The  center  section  (Fig.  26)  is  the  central 

structure  which  con- 
nects  the  upper  wings 

of  a  multiPlane- 

The    principal    part 

of  the  center  section  is 
the  panel  A,  which  is 
•pj    26  a  section  of  wing  and 

the  part  always  missing 

in  a  monoplane  and  sometimes  in  a  biplane.  The  reason  is 
this:  the  monoplane  has  only  one  set  of  wings  and  they  are 
fixed  directly  to  the  fuselage;  the  biplane,  instead,  being  pro- 
vided with  two  sets  of  wings,  has  the  lower  ones  attached  to 
the  fuselage  as  in  a  monoplane,  while  the  upper  wings  are  at- 
tached to  the  center  section,  in  which  case  the  main  parts 
will  be  five;  or  they  are  united  end  to  end  by  means  of  fittings 
and  supported  by  struts,  and  then  there  is  no  center  section, 
in  which  case  the  parts  of  a  biplane  are  also  four  as  in  a 
monoplane. 

The  wooden  parts  of  the  center  section  are  the  struts  B, 
and  the  metallic  parts:  the  cross  bracing  wires  C,  the  bracing 
wires  D  (Fig.  266),  the  drift  wires  E  and  the  antidrift  wires  F. 


AEROPLANE  CONSTRUCTION 


43 


The  cross  bracing  wires  of  the  center  section  are  found 
usually  in  the  front  and  the  rear,  the  bracing  wires  taking 
the  place  of  the  cross  bracing  wires,  which  should  be  on  the 
sides  also.  These  side  cross  bracing  wires  are  eliminated  and 
substituted  by  the  bracing  wires,  because  the  seat  of  the 
aviator  is  almost  always  inside  of  the  center  section  and, 
consequently,  the  sides  must  be  free  from  obstructions,  so 
that  he  can  go  in  and  out. 

The  drift  wires  have  the  object  of  strengthening  the  center 
section  against  the  pressure  of  the  air  when  the  machine  is 


n 


v    A 


A 


<* 


Fig.  27 

in  flight,  and  the  antidrift  wires  are  necessary  to  counteract 
the  tension  of  the  drift  wires  and  to  keep  the  center  section 
straight  and  rigid. 

Wings. — The  wings  (Fig.  27)  are  the  lifting  members  of 
the  machine,  whose  entire  weight  hangs  from  them,  and 
therefore  they  must  be  built  very  strongly. 

The  wooden  parts  of  the  wings  are :  the  spars,  front  A  and 
rear  B;  the  compression  ribs  C,  the  camber  ribs  D  and  the 
false  ribs  E;  the  leading  edge  F  and  the  trailing  edge  G; 
the  wing  tip  H;  the  stringers  /  and  the  three-ply  veneer  J. 
The  metallic  parts  are:  the  drift  wires  K,  the  antidrift  wires 
L  and  the  hinges  or  fittings  M . 


44  AVIATION 

The  wings  are  covered  with  fabric,  which  is  made  tight  by 
coating  it  with  a  special  solution. 

The  compression  ribs  are  solid  and  are  used  for  strength, 
while  the  camber  ribs  are  lightly  built  and  their  purpose  is 
to  give  the  proper  shape  to  the  fabric.  The  false  ribs  are 
merely  strips  of  wood,  which  run  from  the  leading  edge  to 
the  front  spar  and  they  are  used  to  prevent  the  fabric  from 
sinking  between  the  ribs  proper.  Sometimes,  instead  of  the 
false  ribs,  the  three-ply  veneer  is  used,  and  sometimes,  both 
false  ribs  and  three-ply  are  found  in  the  same  wing. 

A  rib  is  made  of  three  parts:  a  web  A  (Fig.  28)  and  two 
cap  strips  B  and  C.  The  web  or  center  part  is  made  in  three 
pieces,  leaving  two  openings  for  the  spars. 


Fig.  28 

The  stringers  are  long  strips  of  wood  running  through  the 
webs  of  the  ribs  to  keep  them  from  rolling  over.  Sometimes, 
these  pieces  are  round  and  they  are  called  dowels. 

Generally,  a  wing  is  cut  toward  the  rear  edge  so  as  to  form 
a  rectangular  plane  N,  which  constitutes  a  part  by  itself  and 
is  hinged  to  the  wing;  this  is  the  aileron.  Sometimes,  the 
part  where  the  aileron  should  be  is  not  cut  off,  but  is  made 
flexible,  so  that  it  can  be  warped  up  or  down,  and  this  is 
especially  the  case  with  the  monoplane. 

The  ailerons  are  used  to  control  the  lateral  stability  of  a 
machine  and  to  bank  it  properly  during  a  turn. 

Empennage. — The  empennage  (Fig.  29)  is  the  tail  of  the 
machine. 

The  parts  of  the  empennage  are:  the  horizontal  stabilizer  A, 
the  vertical  stabilizer  B}  the  elevators  C  and  the  rudder  D. 


AEROPLANE   CONSTRUCTION 


45 


All  these  parts  are  constructed  in  the  same  way  as  the 
wings,  that  is,  they  consist  of  a  framework  of  wood,  braced 
by  wires  and  covered  with  coated  fabric. 


Fig.  29 

The  horizontal  stabilizer  maintains  inherent  longitudinal 
stability  and  the  vertical  stabilizer  inherent  directional 
stability. 

The  elevators  are  used  to  make  a  machine  climb  or  descend 
and  the  rudder  to  make  it  turn  to  the  right  or  left. 

Wires. — When  the  wings  of  an  aeroplane  are  mounted, 
they  are  held  rigidly  in  place  by  means  of  additional  wires, 
struts  and  metallic  tubing. 

In  a  monoplane,  these  additional  bracings  are:  the  cabane 
A  (Fig.  30a),  which  is  a  metallic  framework  built  on  the 
upper  part  of  the  fuselage;  the  landing  wires  B,  which  run 
from  the  top  of  the  cabane  to  the  upper  part  of  the  wings 
and  hold  them  when  the  machine  is  on  land ;  the  flying  wires 
C,  which  run  from  the  undercarriage  to  the  lower  part  of  the 
wings  and  hold  them  when  the  machine  is  in  flight;  and  the 


46 


AVIATION 


drift  wires  D  (Fig.  306),  which  run  from  the  nose  plate  to 
the  wings  and  hold  them  against  the  drift  during  flight. 

The  cabane  is  a  necessity  in  a  monoplane,  because  if  the 
landing  wires  were  attached  directly  to  the  fuselage,  they 


Fig.  30 

would  be  too  low  and  would  hardly  have  any  bracing  strength. 
Even  attached  as  they  are,  these  wires,  and  also  the  flying 
wires,  do  not  give  much  strength  to  the  wings,  running  to 
them  at  a  slant,  and,  consequently,  the  monoplane  is  weak 
in  construction,  although  very  efficient  in  regard  to  lift. 
In  a  biplane  or  multiplane,  the  additional  bracings  are: 


AEROPLANE  CONSTRUCTION  47 

the  interplane  struts  A  (Fig.  31a),  which  hold  the  wings 
apart;  the  landing  wires  B;  the  flying  wires  C;  the  stagger 
and  incidence  wires  D  (Fig.  316),  which  brace  the  wings  one 


Fig.  31 

with  the  other;  and  the  drift  wires  E,  which  run  from  the 
nose  plate  to  the  wings. 

Such  a  box  girder  bracing  makes  the  biplane  very  strong 
in  construction,  but  it  detracts  from  its  lift,  increasing  the 
passive  drift  and  causing  interference  in  the  gap.  For  parity 
of  surface,  the  lift  of  the  biplane  is  about  85  per  cent  that 
of  the  monoplane. 

When  a  biplane  has  an  extension,  further  bracings  are  used 
to  strengthen  it.  These  are:  the  king-post  F  (Fig.  3 la),  the 
bracing  wires  G,  which  serve  as  landing  wires,  and  the  flying 
wires  H. 

The  landing  wires  usually  are  single,  being  sufficient  to 


48  AVIATION 

carry  the  weight  of  the  wings  when  the  machine  is  on  the 
ground  and  to  stand  the  additional  stresses  of  normal  and 
abnormal  landings.  The  flying  wires,  instead,  are  double, 
because  they  must  carry  the  entire  weight  of  the  machine 
and  stand  all  the  abnormal  stresses  imposed  by  the  different 
positions  assumed  by  it  in  flight.  The  important  fact  must 
not  be  overlooked,  however,  that  sometimes  the  landing  wires 
are  subjected  to  the  abnormal  stresses  produced  by  a  re- 
versal of  loading,  which  equal,  if  indeed  they  do  not  surpass, 
those  of  the  flying  wires.  This  takes  place  in  such  cases  as 
steep  gliding,  upside  down  flying  and  rolling,  when  the 
weight  of  the  machine  is  borne  in  part  or  in  full  by  the  top 
of  the  wings  and  transmitted  to  the  landing  wires.  For  these 
performances,  a  machine  must  have  double  landing  wires. 
Power  Plant. — A  very  important  part  entering  in  the 
fl  construction  of  an 

aeroplane  is  the 
power  plant,  which 
to-day  is  the  four- 
stroke  gasoline  mo- 
tor, being  the  light- 
est and  most  pow- 
erful invented  so 
far,  although  it  has 

the   great  inconveniences   of    high    speed,    vibrations   and 
noise. 

The  position  and  number  of  motors  give  different  names 
to  an  aeroplane:  if  the  motor  is  in  front,  the  aeroplane  is  a 
tractor;  if  in  the  rear,  a  pusher;  and  if  the  motors  are  two, 
a  twin-motor  aeroplane,  either  tractor  or  pusher. 

The  tractor  aeroplane  (Fig.  32)  has  the  disadvantage  of  a 
limited  range  of  vision  A  B,  because,  on  account  of  the  motor 
in  front,  the  aviator  has  to  sit  in  the  rear  part  of  the  machine 
to  balance  it,  and  the  wings  limit  his  visual  radius;  but,  on 
the  other  hand,  it  is  less  dangerous  for  the  aviator  in  case  of 
a  fall,  because  the  motor  is  in  front  and  can  not  drop  on  him. 


AEROPLANE  CONSTRUCTION 


49 


The  pusher  (Fig.  33)  has  an  unlimited  range  of  vision, 
because  the  aviator  sits  in  the  front  part  of  the  machine, 
but  in  case  of  a  fall,  the  motor  may  drop  on  him  and  crush 
him  to  death.  Another  great  disadvantage  of  the  pusher 
aeroplane  is  that  it 
needs  outriggers  A  to 
give  clearance  to  the 
propeller.  The  out- 
riggers increase  the 
weight  and  drift,  due 
to  their  size,  position 
and  necessary  bracing 
with  struts  and  wires. 
It  was  just  on  account 
of  this  awkward  con- 
struction that  the  propeller  was  tried  in  front,  although  all 
authorities  agreed  that  for  efficiency  its  best  position  was 
in  the  rear,  but  when  the  first  trial  was  made,  a  most  as- 
tounding result  was  obtained:  the  lift  was  more  than  doubled. 
This  is  due  to  the  fact  that  the  relative  speed  of  the  air  is 
increased  by  being  thrown  back  by  the  propeller  against 
the  machine.  It  is  true  that  the  passive  drift  is  increased, 
also,  and  more  power  is  needed  to  overcome  it,  but  on  the 
other  hand,  the  surface  of  the  wings  can  be  cut  down,  making 
it  possible  to  shorten  the  span  and  increase  the  strength 
of  the  wings,  which  is  a  very  great  advantage,  especially 
in  the  case  of  a  monoplane. 


CONTROLS 

The  controls  are  mechanical  devices  used  to  operate  the 
controlling  planes,  that  is,  the  ailerons,  the  elevators  and 
the  rudder. 

The  controls  in  use  to-day  are  two:  the  wheel  control,  called 
also  Deperdussin  or  Dep,  and  the  stick  control.  In  both 
mechanisms,  the  hands  are  used  to  operate  the  ailerons  and 


50 


AVIATION 


elevators,   which   require   finer  motions,   and   the   feet   for 

operating  the  rudder. 

The  wheel  control  (Fig.  34)  consists  of  a  hand  wheel  A 

(Fig.  34a),  a  drum  B,  a  control  column  C,  two  pulleys  D 

and  a  wire  E.  The 
wheel  is  pivoted  to 
the  upper  end  of  the 
control  column  and 
is  free  to  turn  to  the 
right  and  left,  while 
the  other  end  F  of 
the  control  column  is 
pivoted  to  the  bot- 
tom of  the  cockpit 
and  can  be  moved 
only  in  a  fore  and  aft 
direction.  The  center 
of  the  wire  is  at- 
tached to  the  drum 
at  one  point  and  then 
is  wound  around  a 
spiral  groove  cut  in 
the  side  of  the  drum, 
so  that  if  the  wheel 
is  turned  to  the  right, 
the  left  end  of  the 
wire  is  pulled  and  the 
right  end  released ; 
and  vice  versa.  To 
the  right  end  of  this 


Fig.  34 


wire  is  turnbuckled  a  control  wire  G,  which  after  passing 
around  a  pulley,  runs  to  the  under  side  of  the  right  aileron  H, 
and  to  the  left  end  another  wire  /,  which  goes  to  the  under 
side  of  the  left  aileron  J. 

On  the  upper  side  of  the  wing  with  the  ailerons,  which 
in  a  biplane  is  usually  the  top  wing  alone,  is  a  balance  wire  K, 


AEROPLANE  CONSTRUCTION  51 

whose  right  and  left  end  is  attached  to  the  upper  side  of  the 
right  and  left  aileron  respectively,  so  that  if  one  is  lowered, 
the  other  one  is  raised,  and  vice  versa.  At  the  center  of  the 
balance  wire  is  a  turnbuckle,  which  is  used  to  regulate  the 
position  of  the  ailerons  in  regard  to  the  wings.  All  these 
wires  are  attached  to  masts,  which  are  bolted  to  the  control 
planes. 

With  such  an  arrangement,  a  turn  of  the  wheel  to  the 
right  brings  down  the  left  aileron  and  up  the  right  one,  and 
a  left  turn  of  the  wheel  reverses  the  motions. 

On  the  front  side  of  the  control  column  is  attached  one 
pair  of  wires  L  (Fig.  346),  above  the  pivot  point,  and  on  the 
rear  side,  opposite  the  first  pair,  is  another  one  M.  The 
front  pair  runs  to  the  lower  side  N  of  the  elevators,  the  right 
wire  being  attached  to  the  right  elevator  and  the  left  to  the 
left;  the  rear  pair,  after  passing  around  pulleys  0,  goes  to 
the  top  side  P  of  the  elevators  and  the  two  wires  are  at- 
tached similarly.  The  wires,  therefore,  cross  one  another, 
so  that  a  forward  motion  of  the  wheel  or  control  column 
pulls  down  the  elevators,  and  a  backward  motion  brings 
them  up. 

The  rudder  Q  (Fig.  34c)  is  controlled  by  a  foot  rudder 
bar  R,  pivoted  in  the  center,  and  two  wires,  one  S  attached 
to  the  right  and  the  other  T  to  the  left  end  of  the  bar,  which 
run  to  the  right  and  left  side  of  the  rudder  respectively. 
Thus,  a  push  to  the  right  or  left  end  of  the  bar  pulls  the  rudder 
to  the  right  or  left. 

The  wheel,  therefore,  operates  the  ailerons;  the  column, 
the  elevators;  and  the  foot  rudder  bar,  the  rudder. 

This  control  is  in  a  neutral  position  when  the  wheel  is  so 
turned  that  the  point  of  attachment  of  the  wire  on  the  drum 
is  on  the  top;  the  control  column  vertical  and  the  foot  rudder 
bar  at  right  angles  with  the  longitudinal  axis  of  the  aeroplane. 

Holding  the  control  in  neutral  position,  the  aviator  is 
enabled  to  operate  the  controlling  planes  easily  and,  with 
tae  exception  of  the  rudder,  naturally.  If,  for  instance,  the 


52  AVIATION 

left  wing  dips  down,  the  aviator  instinctively  shifts  his 
body  to  the  right  to  keep  his  balance  and,  in  so  doing,  he 
carries  the  motion  of  the  wheel  to  the  right;  this  brings 
down  the  left  aileron  and  up  the  right.  The  resistance  of 
the  air,  acting  against  the  lower  side  of  the  lower  aileron, 
pushes  the  lower  wing  up,  while  the  resistance  against  the 
upper  side  of  the  upper  aileron  pushes  the  higher  wing  down, 
thus  reestablishing  the  equilibrium.  If  the  case  is  reversed, 
the  same  principle  holds  good.  If  the  aviator  wants  to  come 
down,  he  pushes  the  wheel  down,  the  elevators  go  down,  and 
the  air,  acting  against  the  lower  side  of  the  elevators,  pushes 
the  tail  up  and,  consequently,  the  nose  down;  when  he  wants 
to  go  up,  he  pulls  the  wheel  up  and  the  motion  is  reversed. 
If  he  wants  to  turn  to  the  right  or  left,  he  pushes  the  foot 
rudder  bar  to  the  right  or  left. 

While  the  motion  of  the  rudder  seems  natural,  the  right 
push  of  the  foot  corresponding  to  the  right  turn  of  the  ma- 
chine and  the  left  to  the  left,  it  is  not,  and  we  are  not  used 
to  this  system  of  conrol.  If  we  ride  a  bicycle,  when  we  push 
the  right  side  of  the  handle  bar,  we  turn  to  the  left,  and  vice 
versa.  The  same  should  be  with  the  foot  rudder  bar  and 
it  could  easily  be  accomplished  by  simply  crossing  the  wires. 
Why  this  is  not  done  is  probably  due  to  the  fact  that  the 
aviators,  having  mastered  this  awkward  motion,  are  not 
prone  to  change  it,  fearing  that,  through  force  of  habit,  they 
may  encounter  with  some  accident  in  making  an  involun- 
tary inverse  motion  when  they  are  in  a  dangerous  position, 
and  so  they  teach  the  same  system  to  their  pupils,  continuing 
the  same  erroneous  motions. 

The  stick  control  (Fig.  35)  consists  of  a  vertical  metallic 
tube  or  stick  A,  a  horizontal  tube  B,  two  arms  C  and  two 
bearings  D.  The  stick  is  forked  at  one  end  E  and  is  pivoted 
to  the  horizontal  tube  so  as  to  form  a  universal  joint,  which 
makes  it  possible  to  move  the  stick  from  side  to  side  without 
moving  the  horizontal  tube  and  to  turn  the  latter  in  its 
bearings  when  the  stick  is  pushed  forward  or  pulled  back- 


AEROPLANE   CONSTRUCTION 


53 


ward;  the  arms  are  fixed  to  the  ends  of  the  horizontal  bar;  and 
the  bearings  fastened  to  the  floor  of  the  cockpit.  To  the  stick 
are  fastened  two  wires,  one  to  the  right  F  and  the  other  to 
the  left  side  G,  which  run  to  the  right  and  left  aileron  respec- 
tively, and  to  each 
arm  are  attached  ^^ 

two  wires,  one  at 
each  end,  the  top 
wires  H  (Fig.  356) 
going  to  the  bottom 
of  the  elevators  and 
the  bottom  wires  I 
running  to  the  top 
of  the  elevators; 
these  wires,  there- 
fore, cross  one  an- 
other. 

With  this  mechan- 
ism, a  side  motion 
of  the  stick  oper- 
ates the  ailerons  and 
a  fore  and  aft  motion 
operates  the  elevators, 
in  the  wheel  control. 

While  the  mechanisms  as  described  here  are  not  used  in 
all  machines,  the  changes  are  only  means  to  an  end,  the 
principle  being  always  the  same. 

In  biplanes  having  ailerons  in  both  sets  of  wings,  the 
control  wire  runs  along  the  lower  wings  and  the  motions 
from  the  lower  to  the  upper  ailerons  are  transmitted  by  two 
compensating  wires  or  struts,  one  connecting  the  right  and 
the  other  the  left  ailerons  together.  With  the  use  of  com- 
pensating struts,  there  is  no  need  of  balance  wire,  because 
a  strut  can  be  used  both  to  pull  and  push,  while  a  wire  can 
only  pull. 

Some  machines  have  a  dual  control,  the  two  units  being 


Fig.  35 
The  rudder  system  is  the  same  as 


54 


AVIATION 


connected  by  additional  wires  and  bars  in  such  a  manner 
as  to  make  their  motions  synchronous. 

The  wheel  control  is  slow  in  its  movements  and  is  used 
for  slow  speed  and  training  machines,  while  the  stick  con- 
trol is  operated  quickly  and  is  found  in  speedy  machines 
flown  by  experienced  aviators.  The  stick  can  be  manipu- 
lated with  one  hand  and  some  pilots  have  gone  so  far  as  to 
operate  it  with  their  knees,  thus  leaving  both  hands  free, 
and  as  the  stick  is  round  and  may  slip  from  between  the 
knees,  a  half  round  pad  is  attached  on  each  side  to  fit  the 
legs  and  permit  a  good  hold  on  the  stick.  This  is  a  very 
good  improvement  in  war  machines,  enabling  the  pilot  to 
have  his  hands  free  to  operate  the  gun. 


PONTOONS 

A  special  aeroplane  part  found  only  in  water  machines 
or  hydroaeorplanes  is  the  pontoon  (Fig.  36),  which  is  a  flat 

a.  i> 


Fig.  36 

bottomed,  air-tight,  boat-like  float  attached  to  a  land  ma- 
chine to  enable  it  to  rest  on  and  rise  from  the  water,  skimming 
on  its  surface  like  a  hydroplane. 

A  pontoon  is  usually  built  with  ply-wood,  alternated  with 
painted  canvas,  glued  with  marine  glue  to  a  thickness  of 
about  one-quarter  of  an  inch  and  closely  screwed  to  a  frame- 
work similar  to  that  of  a  boat. 

In  a  pontoon,  we  find:  the  step  A  (Fig.  36a),  the  vent 
pipes  B,  the  bulkheads  C,  the  drain  holes  Z>,  the  hand  holes 


AEROPLANE  CONSTRUCTION  55 

E,  the  reenforcing  struts  F,  the  planing  fins  G  (Fig.  366)  and 
the  battens  H. 

The  step  is  a  very  important  feature  of  the  pontoon  and 
its  object  is  to  facilitate  the  rise  of  the  machine  by  breaking 
the  hold  of  the  water  from  the  bottom  of  the  pontoon. 
When  the  machine  starts  to  skim,  the  inclined  bottom  of 
the  forward  part  of  the  pontoon  presses  the  water  down  and 
makes  it  acquire  a  downward  trend,  which,  on  account  of 
the  adhesion  of 'the  water  to  the  pontoon,  pulls  it  down.  As 
the  step  is  reached,  the  hold  of  the  downward  current  is 
broken  from  the  bottom  and  is  carried  to  the  water  behind 
the  step.  This  water,  being  pulled  downward,  causes  a 
vacuum  behind  the  step,  with  the  result  that  the  pontoon 
can  not  leave  the  water  easily,  if  the  formation  of  the  vacuum 
is  not  prevented  and  this  is  exactly  the  function  of  the  vent 
pipes.  Being  always  open  to  the  air,  they  keep  it  in  con- 
stant contact  with  the  water  at  the  step  and  avoid  the  forma- 
tion of  the  vacuum.  The  diameter  of  these  pipes  when 
first  used  was  about  half  an  inch,  while  now  it  has  reached 
two  and  one-half  inches. 

The  bulkheads  divide  the  pontoon  in  compartments  to 
prevent  it  from  sinking  in  case  of  a  leak,  confining  the  in- 
coming water  to  one  section  only. 

The  drain  holes  serve  to  drain  out  the  water  which  may 
be  found  in  the  pontoon  when  the  machine  is  beached. 
The  drain  holes  are  closed  by  plugs. 

The  hand  holes  are  used  to  bail  out  the  water  while  the 
machine  is  floating,  to  sponge  any  remains  after  it  has  been 
drained  and  to  ventilate  the  pontoon  when  the  machine  is 
on  land.  The  hand  holes  normally  are  closed  by  covers. 

The  reenforcing  struts  strengthen  the  pontoon  against  the 
shock  of  landing,  both  in  a  vertical  and  inclined  direction. 

The  planing  fins  increase  the  planing  surface  of  the  pon- 
toon. The  minimum  surface  should  be  one  square  foot 
for  each  500  pounds  of  weight,  but  in  actuality  is  much  more 
than  that* 


56 


AVIATION 


The  battens  are  attached  to  the  bottom  of  the  pontoon 
to  avoid  damaging  it  when  the  machine  is  beached. 

The  manner  in  which  the  pontoon  is  attached  to  a  ma- 
chine gives  it  a  different  name.  Although  all  water  machines 


Fig.  37 

are  hydroaeroplanes,  this  term  is  used  to  designate  an  aero- 
plane which  has  an  undercarriage  with  one  or  two  pontoons 
attached  to  it  (Fig.  37).  A  machine  which  has  a  pontoon 
in  the  place  of  a  fuselage  is  called  a  flying  boat  (Fig.  38). 

In  regard  to  the  number  of  pontoons  used  in  a  hydro- 
aeroplane, it  is  to  be  noted  that  while  two  pontoons  give 
a  better  support  to  the  machine  on  the  water,  on  the  other 
hand  they  increase  the  drift  when  the  machine  is  in  flight, 
because  for  equal  volume  they  offer  more  surface  than  one 


AEROPLANE   CONSTRUCTION 


57 


pontoon.  Then,  too,  they  subject  the  machine  to  heavy 
stresses  in  a  rough  sea,  because  while  one  pontoon  is  on  the 
crest  of  a  wave,  the  other  is  down  in  the  trough,  and  this 
see-sawing  motion  is  dangerous.  One  pontoon  causes  no 


Fig.  38 

stresses  and  gives  less  drift,  but  is  not  so  steady  and  re- 
quires the  use  of  additional  small  floats  at  the  wing  tips  and 
sometimes  at  the  tail  (Fig.  376). 

Pontoons  offer  a  great  amount  of  lateral  or  keel  surface, 
which  renders  a  water  machine  very  sensitive  to  side  winds 
and  also  causes  it  to  skid  in  a  turning  motion.  To  offset 
this  tendency,  an  extra  plane  or  non-skid  fin  A  (Fig.  38)  is 
attached  to  the  top  of  the  wings,  which,  on  account  of  its 
long  lever  arm,  counterbalances  the  pressure  against  the 
side  of  the  pontoons. 


MATERIALS 

A  successful  aeroplane  must  be  the  combination  of  strength, 
lightness,  rigidity  and  flexibility;  but  to  combine  all  these 
opposite  requirements  into  one  single  structure,  in  order  to 
render  it  indifferent  to  all  kinds  of  stresses,  is  perhaps  the 
most  difficult  problem  man  ever  undertook  to  solve,  which 
calls  into  duty  almost  every  known  branch  of  industry  and 
commands  the  employment  of  first  class  skill  and  material. 


58  AVIATION 

That  a  machine  should  be  strong,  it  goes  without  saying, 
as  it  must  stand  the  pressure  of  the  air,  the  vibrations  of 
the  motor  and  the  shocks  in  starting  and  landing;  that  it 
should  be  light,  it  is  a  capital  requirement  to  accomplish 
flight;  and  rigid  it  must  be  as  a  whole  to  withstand  distortion, 
but  up  to  a  certain  degree,  when  flexibility  comes  in  to  avoid 
undue  stiffness  of  the  parts,  which  might  break  if  stressed 
out  of  proportion;  and  that  these  are  sine  qua  non  conditions 
to  obtain  a  perfect  flying  machine,  nothing  can  attest  more 
than  the  long,  painstaking  work  of  the  great  American 
pioneer,  Professor  Langley,  whose  noteworthy  perseverance 
only  could  bring  the  hard  task  to  completion. 

To  solve  the  difficult  problem  in  the  best  possible  way, 
the  materials  used  to-day  in  constructing  an  aeroplane  are: 
wood,  metal  and  fabric,  variously  combined;  although  the 
all-metal  machine,  which  has  already  made  its  appearance, 
will  undoubtedly  be  the  machine  of  the  future. 

Before  we  examine  in  detail  these  materials,  it  is  necessary 
that  we  know  something  about  their  strength  and  the  terms 
used  in  connection  with  it. 

Strength  of  Materials. — Stress  is  the  load  to  which  a  body 
is  subjected  and  it  is  expressed  in  pounds  per  square  inch. 

Stresses  are  simple  and  compound;  the  simple  are:  com- 
pression, tension  and  shear  stress;  the  compound:  bending 
and  torsion. 

Compression  is  the  stress  which  tends  to  crush  a  body. 

Tension  is  the  stress  which  tends  to  elongate  a  body. 

Shear  stress  is  the  stress  that  tends  to  tear  a  body  in  such 
a  manner  as  to  cause  one  part  to  slide  over  the  other. 

Bending  is  the  combination  of  the  compression  and  ten- 
sion stresses. 

Torsion  is  a  combination  of  the  compression,  tension  and 
sheer  stresses. 

The  bending  stress  has  a  special  importance  in  aeroplane 
construction.  When  a  body  is  bent,  its  molecules  on  the 
outside  curvature  are  under  tension,  while  in  the  inside  they 


AEROPLANE  CONSTRUCTION  59 

are  under  compression  and  in  the  center  none  of  the  two 
stresses  is  felt  and  there  is,  therefore,  a  neutral  line.  This 
enables  us  to  hollow  the  parts  used  in  aeroplanes,  thus  saving 
about  33  per  cent  in  the  weight  of  material. 

All  bodies  have  a  limit  beyond  which  they  can  not  be 
stressed  without  collapsing  and  being  permanently  deformed. 
Strain  is  the  deformation  produced  by  an  over  stress. 

Factor  of  safety  is  the  ratio  of  the  stress  of  collapse  of  a 
body  to  the  maximum  stress  it  is  called  upon  to  withstand. 
If,  for  instance,  a  body  can  stand  a  stress  of- 1000  pounds 
and  it  is  used  to  stand  only  100,  its  factor  of  safety  is  10, 
that  is,  1000  : 100  =  10. 

The  determination  of  the  factor  of  safety  is  a  matter  of 
great  controversy  among  aeroplane  designers,  some  choosing 
a  factor  of  safety  of  6  and  others  going  as  far  as  making  it  15. 
Every  designer  gives  apparently  good  reasons  for  the  factor 
of  safety  adopted,  but  what  really  decides  the  question  is 
the  actual  test,  and  while  it  is  true  that  some  machines  can 
withstand  successfully  all  kinds  of  over  stresses,  it  is  not  less 
true  that  often  they  have  been  burdened  with  unnecessary 
weight.  On  the  other  hand,  while  machines  built  with  a 
rather  low  factor  of  safety  have  given  good  account  of 
themselves  in  the  majority  of  adverse  conditions,  in  some 
cases  they  have  collapsed.  This  was  due  to  the  fact  that 
the  machines  were  built  to  stand  the  most  common  cases  of 
abnormal  stresses,  leaving  out  of  consideration  the  excep- 
tional ones.  Evidently,  this  is  not  a  good  assumption, 
because  the  fact  that  they  do  not  occur  frequently  is  no 
excuse  for  their  exclusion. 

The  calculation  of  the  factor  of  safety  is  based  on  the  case 
of  a  machine  in  horizontal  flight  in  calm  weather,  in  which 
case  the  load  supported  by  the  wings  is  normal  and  equal 
to  the  weight  of  the  machine,  excluding  the  wings,  whose 
weight  is  directly  distributed  over  the  pressure  surface  and 
thus  they  form  the  support  for  the  rest  of  the  machine. 
With  this  as  a  basis,  are  then  calculated  the  greater  stresses 


60  AVIATION 

due  to  the  various  atmospheric  disturbances  and   to  the 
evolutions,  which  a  machine  is  called  upon  to  perform. 

The  air  is  very  far  from  being  a  smooth  and  evenly  flowing 
element;  it  contains  gusts,  eddies  and  upward  and  downward 
trends,  which  constantly  assail  a  machine  from  all  directions 
and  against  which  the  designer  must  provide,  as  he  must 
also  provide  for  the  abnormal  stresses  brought  about  by 
banking,  looping  and  flattening  out,  all  of  which  impose 
upon  the  machine  loadings  considerably  in  excess  of  the 
normal. 

Another  case  to  be  considered  in  figuring  the  factor  of 
safety  is  the  reversal  of  loading  or  top  loading,  when  the 
load  of  the  wings  is  reversed  in  direction  and  exceeds  that 
of  normal  flight. 

A  machine  built  with  a  reasonable  margin  of  safety  will 
remain  sufficiently  under  control  even  if  some  small  structural 
part  breaks  during  flight  and  will  allow  the  aviator  enough 
time  to  land  without  a  smash. 

Everything  considered,  it  would  seem  that  a  factor  of 
safety  of  10  is  well  suited  to  provide  for  all  eventualities. 

Wood. — Wood  is  a  very  unreliable  material,  its  strength 
varying  considerably  with  the  age  of  the  tree,  the  season 
when  it  was  felled,  the  geographical  situation,  the  manner  of 
seasoning,  that  is,  if  natural  or  artificial,  and  the  different 
artificial  method  and  size  of  the  pieces  used  when  seasoned. 
For  this  reason,  the  factor  of  safety  of  wood  is  about  double 
that  of  metal. 

Wood  has  great  tensile  strength  and  generally  is  more 
flexible  than  steel  tubing,  but  one  of  the  chief  obstacles  in 
the  use  of  wood  is  the  difficulty  of  finding  it  in  sufficient 
lengths  without  any  blemishes;  then  it  becomes  necessary 
to  join  the  good  pieces  together  in  laminations,  that  is,  to 
glue  strips  of  wood  alternating  the  joints.  The  glue  in  this 
case  must  be  insoluble,  otherwise  the  strips  will  fall  apart 
through  darrlpness.  A  special  kind  of  lamination  is  the  ply- 
wood, which  is  made  with  very  thin  sheets  or  plies  of  wood 


AEROPLANE   CONSTRUCTION  61 

glued  together  with  the  grain  of  one  ply  running  across 
that  of  the  other,  thus  forming  a  light,  tough  sheet. 

The  woods  most  used  in  aeroplanes  are:  spruce,  ash,  white 
pine,  cedar,  walnut,  mahogany  and  oak. 

Ash  is  a  heavy  wood,  but  it  is  also  very  strong  and  able 
to  stand  all  stresses.  Spruce  is  lighter  than  ash,  but  it  stands 
only  the  stress  of  compression,  and  if  bent  or  twisted,  it 
splits  easily.  White  pine  is  very  light  and  does  not  split. 
Cedar  is  usually  employed  in  the  construction  of  pontoons, 
being  well  able  to  stand  the  action  of  water;  mahogany  is 
also  used  for  pontoons,  but  it  is  very  expensive.  Walnut, 
mahogany  and  oak  are  generally  used  for  propellers;  the 
best  of  the  three  being  mahogany,  as  it  is  the  lightest  and 
has  the  greatest  tensile  strength. 

Each  kind  of  wood  is  used  according  to  its  peculiar  be- 
havior in  regard  to  flexibility,  strength,  lightness  or  hardness, 
but  in  aeroplane  construction,  the  same  kind  of  wood  is  not 
always  used  for  the  same  part  and,  consequently,  only 
general  rules  can  be  given. 

The  spars  are  usually  made  of  ash,  spruce  or  an  ash-spruce 
combination,  and  they  are  hollow  at  the  neutral  axis,  but 
solid  where  the  compression  ribs,  struts  and  wires  are  at- 
tached. 

The  ribs  are  of  spruce  or  of  a  combination  of  spruce  for 
the  cap  strips  and  ash  three-ply  for  the  webs. 

The  leading  and  trailing  edges  are  spruce,  although  metal 
is  more  commonly  used  for  the  latter. 

The  struts  are  generally  spruce. 

The  longerons  and  skids  are  ash. 

The  engine  rails  are  ash  or  laminated  ash  and  spruce. 

No  matter  what  the  kind  of  wood  used,  as  a  general  rule, 
the  parts  of  an  aeroplane  which  must  stand  a  greater  stress 
than  others  are  made  of  stronger,  harder  and  heavier  wood. 
For  this  reason,  the  engine  rails,  which  must  carry  the  weight 
of  the  motor  and  stand  its  vibrations,  must  be  made  of  very 
strong  wood.  The  longerons,  also,  must  be  made  of  strong 


62  AVIATION 

wood,  able  to  stand  all  stresses  without  breaking,  because, 
as  we  have  already  seen,  a  damaged  longeron  means  the 
taking  apart  of  the  fuselage  and,  as  a  consequence,  of  the 
whole  machine. 

Metal. — The  metal  mostly  used  is  steel,  and  that  this  is 
the  best  metal  there  is  no  doubt,  as  it  withstands  successfully 
all  kinds  of  stresses. 

The  main  reasons  against  the  use  of  steel  are:  its  weight, 
its  liability  to  rust,  the  difficulty  of  obtaining  the  point  of 
union  of  the  component  parts  as  strong  as  the  components, 
and  the  fact  that  steel  tubing,  as  generally  used  in  aeroplane 
construction,  while  giving  great  rigidity,  does  not  permit 
of  great  flexibility,  as  in  a  thin  tube  all  the  material  is  at  the 
surface,  far  from  the  neutral  center,  and  if  bent,  it  breaks 
right  through.  For  these  reasons,  wood  has  been  generally 
preferred,  but  the  modern  tendency  is  toward  its  elimina- 
tion, due  to  the  introduction  of  non-rusting  steel  alloys, 
improved  welding  processes  and  more  accurate  calculation 
of  the  strength  of  the  parts.  The  introduction  of  chrome 
nickel  steel  has  increased  the  possibility  of  all  metal  con- 
structions, which  possess  greater  strength  and  homogeneity 
and  permit  the  standardization  of  the  parts. 

Another  non-rusting  alloy  which  is  now  being  tried,  and, 
it  seems,  with  good  results,  is  the  Monel  metal,  which  is  an 
alloy  of  60  parts  of  nickel,  35  of  copper  and  5  of  iron. 

As  the  metal  which  on  account  of  its  lightness  is  the  first 
to  be  thought  of  by  the  aeroplane  builder  is  aluminum,  a 
comparison  between  aluminum  and  steel  is  not  out  of  place. 
While  the  specific  gravity  of  aluminum  is  about  one-third 
that  of  steel,  its  tensile  strength  is  about  one-sixth,  so  that 
in  reality  we  ought  to  use  six  times  an  amount  of  aluminum 
to  have  the  same  tensile  strength  of  steel,  which  consequently 
would  double  the  weight  of  the  material  and  increase  the 
size  of  the  parts,  causing  more  passive  drift.  Then  again 
aluminum  is  not  very  cohesive  and  its  bending  strength 
is  very  bad,  a  fact  which  forbids  its  use,  especially  when 


AEROPLANE  CONSTRUCTION  63 

it  must  be  subjected  to  vibrations.  In  conclusion,  out  of 
the  comparison,  steel  is  the  winner. 

In  aeroplanes,  as  built  to-day,  the  metal  used  is  mostly 
steel  in  the  form  of  wires,  fittings  and  seamless  tubing;  and 
of  such  additional  accessories  as  turnbuckles,  ferrules, 
thimbles,  fair  leads,  bolts,  nuts,  washers,  rivets,  clevis  pins, 
cotter  pins,  tacks,  screws  and  metallic  sheets,  in  whose 
manufacture  other  metals  are  also  employed. 

The  wires  are  solid  and  stranded.  Solid  wire  is  used  in 
parts  of  the  machine  which  are  not  dismantled,  because  a 
solid  wire  can  hardly  be  handled  without  kinking  it  and  a 
kink  means  a  weak  spot,  which,  if  strained,  will  break. 
The  stranded  wire  is  of  three  kinds:  strand  stay,  which 
consists  of  from  7  to  19  wires  stranded  together;  cord  stay, 
which  is  made  with  7  strands  of  from  7  to  19  wires  to  each 
strand;  and  control  cable,  which  consists  of  6  strands  of  7 
wires  each  and  a  cotton  center.  The  strand  stay  and  cord 
stay  are  used  in  all  parts  of  the  machine  which  must  be 
handled  in  assembling,  disassembling  and  adjusting,  because 
they  are  flexible  and  can  easily  be  coiled  without  kinking 
them.  The  control  cable  is  used  for  the  controlling  planes, 
as  it  is  very  flexible  and  can  easily  go  around  pulleys. 

The  fittings  are  made  of  pressed  steel  and  are  used  to  con- 
nect all  the  parts  of  the  machine  and  for  the  attachment  of 
the  wires. 

Seamless  steel  tubing  is  employed  for  edges  of  controlling 
planes,  for  rudder  and  skid  posts,  for  axles,  struts  and  re- 
enforcements. 

Turnbuckles  are  usually  made  with  two  kinds  of  material : 
the  barrel  is  bronze  and  the  shanks  steel.  A  turnbuckle 
(Fig.  39)  is  an  important  little  device,  which  must  be  handled 
with  care,  because  it  is  not  so  strong  as  it  looks.  First  of 
all,  pliers  must  never  be  applied  to  it,  as  the  barrel  is  hollow 
and  may  easily  be  distorted  or  even  cracked  without  showing 
any  outside  sign.  If  it  is  distorted,  the  threads  will  be  spoiled 
and  the  shanks  will  not  work  freely,  and  if  cracked,  it  may 


64 


AVIATION 


Fig.  39 


snap  while  the  machine  is  in  flight  and  cause  a  disaster. 
The  proper  way  to  operate  a  turnbuckle  is  to  insert  a  stiff 
wire  or  nail  in  the  hole  of  the  barrel  and  another  in  the  eye 
of  the  shank  connected  to  the  wire  to  avoid  it  from  turning. 

Whenever  a  turnbuckle  is  un- 
screwed to  disconnect  a  wire, 
the  barrel  must  be  screwed  to 
the  shank  attached  to  the  wire 
for  a  distance  sufficient  to  avoid 
its  occasional  unscrewing  and 
loss.  When  the  wire  is  to  be 
connected  again,  the  barrel 
must  be  unscrewed  all  the  way 
out,  then  screwed  to  the  shank  attached  to  the  fitting  just 
enough  to  catch  it,  the  other  shank  brought  to  the  other  end 
and  then  the  barrel  turned,  thus  tak- 
ing in  both  shanks  evenly.  If  this  rule 
is  not  observed,  one  of  the  shanks 
will  be  all  the  way  in  the  barrel,  coming 
to  the  end  of  its  run,  while  the  other 
shank  is  almost  all  out,  and  this  will  pre- 
vent the  proper  adjustment  of  wires 
which  are  cut  the  right  length  to  be 
properly  tensioned  by  the  turnbuckles. 
After  a  wire  has  been  adjusted,  the 
turnbuckle  must  be  locked  to  avoid  it 
from  turning.  This  is  done  by  insert- 
ing a  wire  A  in  the  hole  of  the  barrel, 
winding  both  ends  around  the  barrel  in 
a  sense  opposite  to  its  unscrewing  di- 
rection, passing  the  ends  through  the 
eyes  of  the  shanks  and  winding  them 
on  the  shanks. 

The  ferrules  are  of  two  kinds:  solid  and  wire.  A  solid 
ferrule  (Fig.  40a)  is  a  short  tube,  usually  of  copper,  flattened 
enough  to  make  it  oval.  A  wire  ferrule  (Fig.  406)  is  made 


AEROPLANE   CONSTRUCTION 


65 


with  a  wire  coiled  in  a  spiral  and  flattened  to  become  oval. 
These  ferrules  are  used  in  connection  with  the  looping  of  a 
solid  wire. 

A  thimble  (Fig.  41)  is  an  almond-shaped  steel  eye  used  in 
the  inside  of  a  stranded  wire  loop,  to  protect  it  from  the  wear 
caused  by  the  fric- 
tion of  the  shank. 

A  loop  is  the  doub- 
ling of  a  wire  in  such 
a  manner  as  to  form 


Fig.  42 


an  eye  for  the  reception  of  a  turnbuckle  shank  or  the  pin 
or  rivet  of  a  fitting. 

With  a  solid  wire,  only  one  kind  of  loop  can  be  made, 
that  is,  the  ferrule  and  loop  (Fig.  42),  which  is  fastened  by 
means  of  a  ferrule  and  by  bending  the  end  of  the  wire. 


Fig.  43 

With  a  stranded  wire,  three  kinds  of  loops  can  be  made: 
the  wrapped  and  soldered  loop,  the  thimble  and  loop  wrapped 
and  soldered  and  the  spliced  loop.  The  wrapped  and  soldered 
loop  (Fig.  43)  is  made  by  winding  fine  copper  wire  around 


Fig.  44 

the  stranded  wire  at  the  part  where  it  is  to  be  looped,  to 
protect  it  from  wearing,  doubling  it,  continuing  to  wrap 
both  the  wire  and  its  doubled  end  and  soldering  the  entire 
loop.  The  thimble  and  loop  wrapped  and  soldered  is  made 
by  inserting  a  thimble  in  the  loop  (Fig.  44),  wrapping  its 


66  AVIATION 

end  with  copper  wire  and  soldering  loop  and  thimble.  The 
spliced  loop  (Fig.  45)  is  made  in  a  way  similar  to  the  thimble 
and  loop,  with  the  difference  that  the  end  of  the  wire,  instead 
of  being  soldered,  is  unwound  and  its  component  strands 


Fig.  45 

inserted  repeatedly  in  the  wire  and,  finally,  served  with  a 
fine  string  or  wire  for  further  protection. 

Of  the  loops  made  with  a  stranded  wire,  the  spliced  loop 
is  to  be  preferred  for  the  absence  of  solder,  because  the 
soldering  process  requires  the  employment  of  acid,  which 
filters  into  the  wire  and  in  due  time  will  corrode  it,  causing 
it  to  break  when  the  least  expected  and  bringing  about  acci- 
dents. The  spliced  loop  weakens  the  wire  at  the  end  of  the 
splice,  due  to  its  kinking  in  splicing,  but  this  loop  will  at 
all  times  give  a  warning  of  its  weak  condition  and  can  be 
replaced  in  time  to  avoid  damage. 

Fair  lead  (Fig.  46)  is  a  short  copper  tube  with  the  ends 
enlarged    funnel-like    to    prevent   its    edges 
from  cutting  and   is  used  for  the  passage 
and  guide  of  control  cables. 

The  bolts  used  in  aeroplanes  have  a  hole 

FIE  46 

at  the  point  (Fig.  47a)  for  the  introduc- 
tion of  a  locking  cotter  pin;  the  nuts  are  usually  castel- 
lated (Fig.  476),  that  is,  they  have  grooves  at  their 
upper  face  to  receive  a  cotter  pin;  the  washers  (Fig.  47c) 
are  disks  with  a  hole  in  the  center  for  the  passage  of  a 
bolt;  the  rivets  (Fig.  47 d)  are  short  bolts  without  thread, 
used  to  lock  parts  together  by  burring  the  edge  of  the 
point;  clevis  pins  (Fig.  47e)  are  rivets  with  a  hole  at  the 
point  for  the  passage  of  a  cotter  pin;  the  cotter  pins  (Fig.  47/) 


AEROPLANE   CONSTRUCTION 


67 


are  split  keys  made  by  bending  a  half  round  wire  with  the 
flat  face  inside,  so  as  to  form  an  eye  at  the  bend  and  bring 
together  the  two  halves  or  leaves,  which  thus  make  a  round 
wire  open  in  the  middle,  and  they  are  used  in  holes  of  clevis 


Fig.  47 

pins  to  lock  them  by  spreading  out  the  leaves  or  to  lock  the 
nuts  of  bolts  provided  with  an  opposite  hole  at  the  threaded 
end.  All  these  parts  are  made  of  steel. 

The  tacks,  usually  barbed  (Fig.  47gr),  and  the  screws  are 
copper  or  brass  and  are  used  to  fasten  fabric  on  wood  or 
wooden  parts  together. 

Metallic  sheets  of  aluminum  or  galvanized  tin  are  usually 
employed  for  stream-lining  purposes. 

Fabric. — The  best  fabric  used  for  covering  the  planes  is 
unbleached  Irish  linen,  because  the  thread  of  the  flax,  which 
is  used  to  make  it,  is  about  two  feet  long  and  thus  provides 
a  good  overlapping  margin  when  it  is  spun,  forming  a  very 
strong  cloth.  It  is  left  in  its  natural  color  to  be  stronger, 


68  AVIATION 

because  the  bleaching  chemicals  weaken  the  fabric.  Irish 
linen  weighs  about  4  ounces  per  square  yard  and  stands  a 
load  of  over  60  pounds  per  linear  inch  of  warp.  Sometimes 
cotton,  silk  or  a  combination  of  both  is  used  for  plane  cover- 
ing, as  cotton  alone  does  not  make  a  strong  fabric,  its  threads 
being  very  short,  and  silk,  although  light  and  very  long 
threaded,  has  the  fault  of  absorbing  moisture,  besides  being 
very  expensive.  Sea  Island  cotton  is  also  used  with  good 
results,  its  thread  being  about  as  long  as  that  of  Irish  linen. 

There  are  two  methods  in  use  for  covering  the  wings  with 
fabric.  One  consists  in  throwing  a  large  sheet  of  cloth  on 
the  frame,  tacking  it  temporarily  on  one  edge,  passing  the 
cloth  around,  tacking  it  permanently  in  place  at  all  the  edges 
and  ribs  and  sewing  it  around  the  ribs.  The  other  method  is 
to  be  preferred,  because  it  is  easier  and  gives  better  results. 
It  consists  in  cutting  the  fabric  in  the  shape  of  the  wing, 
sewing  the  edges  around  and  turning  it  inside  out,  forming 
a  kind  of  a  bag,  in  which  the  frame  is  slipped  and  the  mouth 
of  the  bag  tacked  permanently  at  the  root  end  of  the  wing. 
The  fabric  is  then  tacked  with  as  few  tacks  as  possible  on 
the  under  camber  of  the  ribs.  The  wing  is  stood  on  the 
leading  edge,  additional  strips  of  fabric  laid  on  both  sides  of 
the  ribs  and  both  strips  and  fabric  sewed  around  the  ribs 
with  about  four  loops  of  thread,  which  are  repeated  at  a 
distance  of  a  couple  of  inches.  This  gives  the  proper  shape 
to  the  fabric,  but  does  not  make  it  very  ti^ht  and  to  accom- 
plish this  result,  it  is  painted  with  dope,  and  then  an  extra 
strip  of  cloth  doped  on  each  side  of  the  ribs  to  cover  the 
stitches. 

Dope  is  cellulose  nitrate  or  acetate  dissolved  in  banana 
oil.  Cellulose  nitrate  is  the  same  compound  of  guncotton 
and  is  therefore  highly  inflammable,  while  the  acetate  is 
less  inflammable.  Banana  oil  is  a  mixture  of  acetone  and 
amylacetate  with  liquid  celluloid.  As  the  vapor  of  the  solv- 
ent is  inflammable  and  volatile,  care  should  be  taken  not 
to  have  an  open  fire  in  close  vicinity  of  the  dope  container 


AEROPLANE   CONSTRUCTION  69 

and  not  to  leave  it  uncovered  or  the  dope  will  thicken,  due 
to  the  evaporation  of  the  solvent.  Dope  will  thicken  also 
at  a  low  temperature.  If  the  thickening  is  due  to  evapora- 
tion, the  dope  can  be  brought  to  its  normal  flow  by  adding 
the  right  amount  of  solvent,  but  if  caused  by  the  tempera- 
ture, it  is  only  necessary  to  warm  it  for  a  short  time  by  putting 
the  container  in  some  warm  place. 

The  brushes  used  for  the  dope  must  be  kept  immersed  in 
it  or  they  will  stiffen,  in  which  case  it  is  necessary  to  stand 
them  in  it  until  they  become  soft  again. 

If  the  fabric  to  be  doped,  as  well  as  the  brushes  and  dope 
cans,  are  not  clean,  dry  and  free  from  oil  or  grease,  the  dope 
will  not  adhere  well  and  will  peel  off  when  dry.  Fabric  soiled 
with  greasy  matter  can  be  easily  cleaned  by  rubbing  it  with 
a  piece  of  cloth  moistened  with  gasoline  or  acetone. 

In  doping  new  fabric,  the  first  coat  is  applied  with  a  light 
pressure  on  the  brush  to  cause  the  dope  to  filter  through,  but 
all  successive  coats  must  be  applied  lightly  and  quickly  with- 
out brushing  out  as  is  usual  with  paint,  otherwise  the  previous 
coatings  will  be  cut.  Every  coat  must  be  dry  and  scraped 
with  steel  wool  to  even  up  the  dope  before  the  next  coat  is 
applied.  Four  coats  of  dope  are  required  to  make  the  fabric 
very  tight  and  airproof.  To  render  the  dope  less  inflammable 
and  make  it  waterproof,  two  coats  of  spar  varnish  are  given. 

Spar  varnish  is  a  dense,  but  clear  and  easy  flowing,  yellow 
liquid,  which  dries  quickly  with  a  lasting  luster.  It  is  the 
best  kind  of  varnish  for  exterior  work,  being  waterproof, 
elastic,  durable  and  able  to  stand  the  effects  of  grease,  oil, 
rain,  hot  and  cold,  fresh  or  salt  water,  extreme  or  sudden 
variation  of  temperature,  without  cracking  or  changing  its 
color,  and  for  these  reasons,  it  is  used  on  boats  as  a  protective 
coating  for  wood,  metal  and  fabric. 

A  change  of  color  in  a  varnish  is  a  sign  of  deterioration, 
because  it  then  becomes  porous  and  allows  the  harmful 
elements  to  filter  through  and  attack  the  materials  which 
it  is  intended  to  protect. 


CHAPTER  III 
RIGGING 

ASSEMBLING 

Fuselage. — To  assemble  a  machine,  the  first  thing  to  do 
is  to  place  the  fuselage  in  the  position  necessary  to  attach  to 
it  all  the  other  main  parts.  To  this  effect,  the  tail  skid  is 
connected  by  pinning  it  to  the  socket  of  the  tail  post  or  in- 
dependent skid  post  and  tying  the  shock  absorber.  The 
front  part  of  the  fuselage  is  then  lifted  on  a  wooden  horse 
high  enough  to  allow  the  undercarriage  to  be  fitted  to  it. 
In  lifting  the  fuselage,  care  must  be  taken  not  to  damage  it, 
and  if  block  and  tackle  is  used,  the  hook  of  the  block  must 
be  attached  to  a  line  passed  under  the  engine  rails. 

Undercarriage. — After  mounting  the  wheels  on  the  axle, 
the  undercarriage  is  pushed  under  the  engine  section  of  the 
fuselage  until  the  fittings  correspond  to  the  struts,  which  are 
put  in  place  and  bolted.  The  cross  bracing  wires  are  then 
connected  and  the  fuselage  tail  is  raised  and  supported  on  a 
horse,  so  as  to  assume  the  rigging  position.  The  shock  ab- 
sorbers are  wrapped  in  place  and  tied. 

Center  Section. — The  center  section  struts  are  bolted  in 
their  sockets  on  the  fuselage,  the  panel  mounted  and  bolted 
to  the  struts,  and  the  cross  bracing  wires  and  drift  and  anti- 
drift  wires  attached  in  place. 

Wings. — The  ailerons  are  removed  to  prevent  any  possible 
damage  and  facilitate  the  work,  and  the  wings  assembled 
in  pairs  by  standing  them  on  the  leading  edges,  which  must 
rest  on  cloth  or  cushions  to  avoid  damaging  the  fabric  on 
the  nose  of  the  wings.  The  two  wings  are  spaced  apart  the 
proper  distance,  the  front  and  rear  struts  bolted  in  their 

70 


RIGGING  71 

sockets,  the  stagger  and  incidence  wires  attached  first,  to 
prevent  the  wings  from  wabbling,  and  then  the  front  and 
rear  flying  and  landing  wires  are  connected  to  make  the 
structure  rigid.  The  wings  are  now  lifted  bodily  and  con- 
nected to  the  center  section  by  means  of  the  top  and  bottom 
hinges  and  pins  and  the  inner  landing  and  flying  wires.  The 
first  pair  of  wings  must  be  supported  by  a  horse  placed  di- 
rectly under  the  outer  struts  until  the  second  pair  is  con- 
nected. The  ailerons  are  then  mounted  by.  means  of  the 
hinges  and  pins. 

Great  care  must  be  taken  in  handling  the  wings  to  avoid 
damage  and  they  must  never  be  lifted  by  taking  hold  of  the 
struts  or  trailing  edges.  The  best  way  is  to  lift  them  by 
means  of  wooden  boards,  placing  blocks  between  the  boards 
and  the  spars,  which  thus  carry  the  load. 

The  top  pins  are  put  in  place  first,  because  they  are  enough 
to  hold  the  wings  and  in  the  meantime  facilitate  the  intro- 
duction of  the  lower  pins.  The  front  struts  and  flying  and 
landing  wires  are  attached  before  the  rear  ones  to  make  the 
work  easier,  as  the  latter  would  be  in  the  way,  if  they  were 
put  in  place  first. 

In  dismantling  the  wings,  the  mounting  process  is  re- 
versed, by  starting  to  detach  first  the  part  that  was  attached 
last. 

To  facilitate  the  assembling  of  the  wings  and  prevent 
errors  in  mounting,  the  struts  are  numbered,  and  although 
the  system  varies  with  different  manufacturers,  the  numbers 
are  always  painted  on  the  inside  part  of  the  struts,  to  enable 
the  aviator  to  see  them  from  his  seat  and  easily  detect  any 
error  of  position  or  inversion. 

As  a  general  rule,  when  a  part  has  been  assembled,  the 
nuts  of  the  bolts  are  screwed  tight,  cotter  pinned  and  the 
leaves  of  the  pins  spread  backward.  The  hinge  pins  are 
also  cotter  pinned  in  the  same  way. 

When  possible,  the  bolts  are  put  in  place  with  the  point 
downward,  so  that  if  a  nut  should  come  loose,  the  bolt  would 


72  AVIATION 

not  fall;  although  this  must  not  be  an  excuse  for  carelessness 
on  the  part  of  the  rigger  to  omit  the  pinning  of  the  bolts. 
Whenever  the  position  of  a  bolt  is  not  vertical  or  inclined, 
so  that  this  rule  can  not  be  followed,  then  the  nut  is  placed 
on  the  inside,  to  be  easily  seen  by  the  aviator  from  his  seat. 

Empennage. — The  horizontal  stabilizer  is  bolted  in  place 
and  the  bracing  wires  or  struts  attached  on  both  sides.  The 
vertical  stabilizer  is  bolted  on  the  horizontal  stabilizer  and 
the  bracing  wires  attached  on  both  sides.  The  rudder  is 
mounted  on  the  rudder  post  by  means  of  the  hinges  and 
hinge  pins.  The  elevators  are  connected  with  the  horizontal 
stabilizer  in  the  same  way  as  the  rudder. 

Control  Wires. — The  control  wires  of  the  ailerons,  ele- 
vators and  rudder  are  connected  by  means  of  the  turn- 
buckles. 

TRUING 

In  truing  up  an  aeroplane,  the  basis  of  all  adjustments 
is  the  manipulation  of  the  cross  bracing  and  opposite  wires, 
that  is,  the  slackening  of  one  and  the  tightening  of  the  other 
to  properly  reshape  parts  thrown  out  of  true. 

To  facilitate  the  work,  the  angles  are  measured  in  inches 
instead  of  in  degrees,  as  they  ought  to  be  measured. 

The  right  and  left  side  of  an  aeroplane  and  the  clockwise 
and  anticlockwise  revolution  of  a  propeller  are  determined 
by  the  position  of  the  aviator  sitting  in  the  machine. 

The  following  rules  for  truing  up  an  aeroplane  are  in- 
tended for  field  shop  work,  which  is  quite  different  from  that 
done  in  the  factory,  where  the  rigger  has  at  his  disposal  all 
the  necessary  equipment  and  tools  to  obtain  the  best  results 
in  the  least  time. 

Fuselage. — To  true  up  an  aeroplane,  the  first  thing  to  do 
is  to  place  the  fuselage  in  the  rigging  position  (Fig.  48), 
which  is  done  by  leveling  the  engine  rails  longitudinally  and 
laterally,  as  they  are  the  basis  of  the  alignment  of  all  parts. 
To  accomplish  this,  a  horse  is  placed  under  the  fuselage, 


RIGGING  73 

immediately  in  the  rear  of  the  engine  section,  so  that  the 
center  of  gravity  of  the  fuselage  will  be  toward  its  rear  and 
the  tail  will  have  a  tendency  to  fall  to  the  ground.  While 
the  tail  is  being  supported  temporarily,  a  chain  is  tied  to  the 


Fig.  48 

nose  plate  and  to  an  eye  fixed  to  the  floor.  This  will  keep 
the  fuselage  in  place  with  the  tail  sticking  out  unsupported. 
To  facilitate  the  work,  the  chain  has  a  turnbuckle  to  raise 
and  lower  the  fuselage  any  desired  amount.  A  level  is  now 
placed  laterally  on  both  engine  rails  and  the  fuselage  leveled 
crosswise  by  inserting  a  wooden  wedge  between  the  fuselage 
and  the  top  rail  of  the  horse  at  the  side  which  needs  to  be 
raised.  This  done,  the  level  is  placed  longitudinally  on  one 
of  the  engine  rails  and  the  fuselage  leveled  lengthwise,  moving 
it  up  or  down  by  means  of  the  turnbuckle.  The  level  is  then 
placed  on  the  other  engine  rail,  which,  if  it  is  true,  should 
also  be  level;  if  it  is  not,  it  must  be  adjusted  by  means  of  the 
side  cross  bracing  wires  of  the  engine  section,  loosening  one 
and  tightening  the  other  the  necessary  amount.  Then,  the 
centers  of  the  bottom  struts  of  the  engine  section  are  marked, 
a  line  stretched  from  the  center  of  the  first  to  that  of  the 
last  strut  and  the  bottom  cross  bracing  wires  manipulated 
until  the  line  cuts  all  center  marks.  The  next  thing  is  to 
level  the  fuselage  from  the  rear  of  the  engine  section  to  the 
tail.  To  do  this,  all  the  internal  cross  bracing  wires  must 
first  be  slackened,  otherwise  they  bind  the  manipulation  of 


74  AVIATION 

the  other  wires;  then  both  longerons  are  leveled  longitudin- 
ally by  sighting  them  and  correcting  roughly  by  eye  any 
up  and  down  distortions  by  manipulating  the  cross  bracing 
wires,  after  which  the  level  is  placed  longitudinally  on  the 
longerons  to  straighten  them  out  properly  by  the  use  of  the 
same  wires. 

To  correct  any  sidewise  distortion,  the  top  and  bottom 
cross  bracing  wires  are  used  respectively.  This  work  is 
done  by  measuring  and  marking  the  centers  of  all  top  and 
bottom  struts  and  stretching  lines  from  the  centers  of  the 
last  top  and  bottom  struts  of  the  engine  section  to  the  rudder 
post,  and  adjusting  the  top  and  bottom  cross  bracing  wires 
until  the  lines  cut  all  the  center  marks  on  the  struts. 

The  internal  cross  bracing  wires  are  now  tightened,  while 
the  level  is  placed  transversally  on  the  longerons  to  avoid 
throwing  them  out  of  level  in  tightening  the  wires  improperly. 

The  above  rules  hold  true  with  a  machine  having  the  line 
of  thrust  parallel  with  the  top  longerons;  if  this  is  not  the 
case,  special  rules  must  be  furnished  by  the  manufacturer. 

The  assumption  has  also  been  made  that,  the  motor  is 
not  in  place  on  its  bed ;  if  it  is,  then  the  level  must  be  placed 
on  any  available  part  of  the  engine  rails,  and,  if  it  need  be, 
even  held  against  their  bottom. 

Undercarriage. — In  case  of  a  new  machine,  the  front  and 
rear  cross  bracing  wires  must  be  manipulated  until  they 
have  the  same  length  and  tension.  With  an  old  machine, 
this  system  can  not  be  applied,  as  the  fittings  may  be  dis- 
torted or  the  heads  of  the  bolts  sunk  unevenly  into  the  wood, 
and  the  length  of  the  wires  may  not  be  the  same,  although 
the  undercarriage  may  be  aligned. 

A  method  applicable  to  all  machines  is  to  mark  the  center 
of  the  fuselage  at  a  point  directly  above  the  axle  of  the  wheels 
and  the  center  of  the  axle  or  the  spreader  (Fig.  49),  then  to 
drop  a  plumb  line  from  the  upper  mark  and  adjust  the  cross 
bracing  wires  until  the  point  of  the  bob  is  over  the  center  of 
the  axle.  If  there  is  no  part  available  on  the  fuselage  to 


RIGGING 


75 


mark  its  center,  a  yard  stick  may  be  laid  on  the  longerons 
and  the  center  taken  from  there. 

Center  Section.  —  The  center  section  is  trued  up  in  a  way 
similar  to  that  of  the  undercarriage,  that  is,  by  marking 
the  centers  of  the  leading  and  trailing 
edges  or  front  and  rear  spars  of  the 
center  section  panel  and  the  centers  of 
the  struts  below  them  on  the  fuselage 
or  by  using  yard  sticks  in  the  absence 
of  struts  (Fig.  50a).  The  center  sec- 
tion is  then  aligned  by  manipulating 
the  front  and  rear  cross  bracing  wires 
until  the  points  of  the  bobs  of  the 
plumb  lines  dropped  from  the  upper  center  marks  are  di- 
rectly above  the  lower  ones. 

If  the  machine  has  a  stagger,  a  plumb  line  is  dropped  from 
the  leading  edge  of  the  center  section  panel  in  front  of  each 
strut  (Fig.  506)  and  the  measurements  taken  in  front  of 
the  bottom  sockets  of  the  same  struts,  adjusting  the  drift 
and  antidrift  wires  until  the  proper  distances  are  obtained. 


-  49 


Fig.  50 


Wings. — Leading  edg^.  To  make  any  adjustment  in  the 
wings,  the  first  thing  to  do  is  to  slacken  the  stagger  and 
incidence  wires,  otherwise  they  bind  the  manipulation  of 


76 


AVIATION 


the  other  wires.  This  done,  the  leading  edges  of  the  wings 
are  aligned  by  standing  on  a  ladder  some  distance  away 
from  the  machine,  sighting  along  the  leading  edge  of  each 
top  wing  separately  and  straightening  it  by  means  of  the 
front  landing  and  flying  wires  of  the  outer  bay.  The  manipu- 
lation of  these  wires  straightens  also  the  leading  edges  of 
the  lower  wings. 

When  a  machine  has  an  overhang,  its  wires  must  be  used 
too  in  the  straightening  process. 

After  the  wings  are  straightened,  they  must  be  brought 
in  line  with  the  leading  edge  of  the  center  section  panel  by 
manipulating  the  landing  and  flying  wires  of  the  inner  bay. 

The  reason  why  the  outer  bay  wires  are  used  to  straighten 
the  wings  is  because  one  end  of  each  wire  is  attached  to  the 


Fig.  51 

spar  of  the  upper  wing  and  the  other  end  to  the  spar  of  the 
lower  wing  between  struts,  thus  bracing  the  rectangular 
framework  formed  by  the  spars  and  struts  and  rendering 
possible  their  adjustment;  while  only  one  end  of  each  wire 
of  the  inner  bay  is  fixed  to  the  spar,  the  other  being  fastened 
to  the  center  section  or  the  fuselage,  making  possible  only 
the  raising  or  lowering  of  the  wings.  In  the  entire  straighten- 
ing and  aligning  process,  only  front  wires  are  used,  because 
the  rear  are  manipulated  to  set  the  angle  of  incidence. 

Lateral  Dihedral  Angle. — To  set  the  lateral  dihedral  angle, 
one  end  of  a  line  is  tied  to  the  outer  front  strut  of  one  wing 


RIGGING  77 

(Fig.  51),  then  passed  over  both  wings  along  the  front  spar, 
stretched  enough  to  avoid  sagging  and  tied  to  the  outer 
front  strut  of  the  other  wing.  The  measurement  is  taken 
from  the  line  to  both  sides  of  the  center  section  panel,  and 
the  landing  and  flying  wires  of  the  inner  bay  are  manipulated 
until  the  proper  distance  is  obtained. 

Care  must  be  taken  to  measure  from  both  sides  of  the 
center  section,  because  if  the  distance  is  measured  at  the 
center,  the  dihedral  angle  may  be  set  wrong,  as  the  wings 
may  not  have  been  raised  equally  on  both  sides  and  while 
one  is  higher  than  the  other,  the  center  may  give  the  proper 
distance. 

The  hidedral  angle  may  be  checked  by  taking  measure- 
ments from  the  center  of  the  leading  edge  of  the  center 
section  panel  to  equal  points  on  both  sides  of  the  wings; 
for  instance,  from  that  center  mark  A  to  the  lower  sockets 
of  the  inner  struts  B,  B',  or  the  outer  struts  C,  C".  If  the 
dihedral  is  right,  each  pair  of  distances  must  be  equal, 
A  B  =  A  B',  A  C  =  A  C'. 

Angle  of  Incidence. — The  angle  of  incidence  is  checked 
at  the  root  end  of  the  wings  and  measured  under  each  set 
of  struts  by  placing 
against  the  center 
of  the  rear  spar  one 
end  of  a  straight  F. 

edge  (Fig.  52),  level- 
ing it  out  and  measuring  vertically  from  the  center  of  the 
front  spar  to  the  top  of  the  straight  edge.  The  proper  dis- 
tance can  be  given  by  manipulating  the  rear  flying  and  land- 
ing wires.  The  front  landing  and  flying  wires  must  not  be 
tou.ched,  as  this  would  throw  off  the  adjustment  of  the 
wings,  both  in  regard  to  straightness  and  alignment  with 
the  center  section  panel  or  dihedral  angle,  if  any. 

The  angle  of  incidence  is  never  measured  from  the  trailing 
edge  or  between  the  struts,  as  the  possible  warping  of  these 
parts  may  give  a  wrong  measurement. 


78  AVIATION 

Stagger. — The  stagger  is  set  by  dropping  plumb  lines 
from  the  leading  edges  of  the  upper  wings  in  front  of  each 
strut  (Fig.  53)  and  making  the  distance  specified  equal 
throughout,  by  measuring  from  the  lower  edges  to  the  plumb 

lines  and  manipulat- 
ing the  stagger  and 
incidence  wires  ac- 
cordingly. 

Care  must  be  taken 
in    determining 
whether  the  specified 
distance  is  to  be  meas- 
ured along  the  chord  or  a  horizontal  distance  straight  out, 
as  this  causes  a  difference  which  is  enough  to  unbalance  a 
machine. 

Wash  in  and  wash  out. — To  correct  the  direct  effect  of  the 
propeller  torque,  the  angle  of  incidence  is  changed  usually 
at  the  wing  tip  under  the  rear  outer  strut,  but  in  some  ma- 
chines it  is  tapered  down  from  the  tip  to  the  root  of  the 
wing  by  adjusting  the  angle  under  both  rear  struts. 

Aileron  Droop. — The  ailerons  are  drooped  by  lengthening 
the   balance   wire   and   taking   the   required   measurement 
from  the  bottom  of  the  rear  edge  of 
the  wing  to  the  bottom  of  the  rear 
edge  of  each  aileron.    The  droop  is 
given  for  the  reason  that  when  the 


machine  is  in  flight,  the  pressure  of 

the  air  under  the  ailerons  takes  up  ^\J_|J-^'^ 

the  slack  and  brings  them  in  line 

with  the  wings. 

Empennage. — The  horizontal  stabilizer  A  (Fig.  54)  must 
be  in  a  horizontal  position,  which  is  determined  by  bolting 
it  in  place  properly  and  adjusting  its  side  bracing  wires  B 
andC. 

The  vertical  stabilizer  D  (Fig.  54)  is  aligned  by  adjusting 
its  side  bracing  wires  E  and  F  until  it  is  vertical. 


RIGGING 


79 


c 

Fig.  55 


With  the  control  column  in  its  neutral  position,  the  control 
cables  are  adjusted  until  the  elevators  A  (Fig.  55)  are  on 
a  line  with  the  horizontal  stabilizer  B,  using  for  this  a  straight 
edge  C  under  them 
both  or  sighting 
them. 

With  the  foot  rud- 
der bar  in  its  neutral 
position,  the  control  wires  are  adjusted  until  the  rudder  A 
(Fig.  56)  is  at  right  angles  with  the  rear  edge  ef  the  horizon- 
tal stabilizer  B. 

The  rules  regarding  the  rudder  and  elevators  do  not  take 
into  consideration  the  correction  of  the  indirect  effect  of  the 
propeller  torque,  nor  the  drooping  of  the  elevators;  if  these 
adjustments  must  be  made,  the  following  process  is  available: 

If  the  propeller  torque  is  corrected  by  means  of  the  vertical 
stabilizer  A  (Fig.  57),  the  rigger  has  nothing  to  do  with  it, 
its  mere  bolting  in  place  giving  the  necessary  position. 

To  set  the  rudder  at  an  angle  to  the  right  A  (Fig.  58)  or 


Fig.  57 


left,  a  line  B  C  is  stretched  from  the  vertical  stabilizer  D 
to  the  rudder  and,  keeping  the  foot  rudder  bar  neutral,  the 
control  cables  are  so  adjusted  as  to  make  one  so  much  shorter 


80 


AVIATION 


than  the  other  that  the  rudder  will  form  an  angle  a  with  the 
line,  the  given  distance  being  measured  from  the  rear  edge 

of  the  rudder  to  the  line.  A 
straight  edge  may  be  used  in- 
stead of  a  line. 

To  droop  the  elevators,  a 
line  A  B  (Fig.  59)  is  stretched 
over  the  top  and  on  each  side 
of  the  horizontal  stabilizer  C, 
in  such  a  way  as  to  follow  its 
camber,  and  the  droop  meas- 
ured from  the  upper  side  of  the 
rear  edge  of  each  elevator  D  to 
the  line. 

As  a  general  rule,  whenever 
an  aeroplane  part  is  trued  up, 
the  turnbuckles  are  locked. 

Controls. — The  control  cables 
must  be  so  adjusted  that  by  moving  the  controls  sharply 
about  1/8  of  an  inch,  the  motion  must  be  transmitted  to 
the  controlling  planes  without  stiffness  or  slack.  The  control 


Fig.  59 

mechanism,  pulleys  and  hinges,  as  all  other  moving  parts 
of  the  machine,  must  be  well  lubricated  with  graphite  to 
work  freely  and  smoothly. 

RIGGING  CARE  AND  FAULTS 

The  greatest  care  must  be  exercised  in  handling  all  parts 
of  the  machine  in  assembling  and  truing  them  up,  to  avoid 
damage,  and  in  adjusting  them  properly. 

In  truing  up  an  aeroplane,  a  great  deal  depends  on  the 
perfection  of  the  tools  used  and  the  way  they  are  handled. 


RIGGING  81 

The  adjustable  end  wrenches,  for  instance,  must  have  the 
right  opening  and  the  fixed  end  wrenches  must  be  the  proper 
size  in  operating  the  nuts,  otherwise  they  will  round  their 
edges  and  spoil  them.  The  levels  and  straight  edges  must 
be  perfect,  and  to  make  sure  of  this,  they  must  be  tried 
before  using  them.  To  this  end,  the  straight  edge  is  clamped 
lightly  in  a  vise  and  leveled,  then  the  level  is  reversed  on 
exactly  the  same  place  and  the  bubble  watched  carefully  to 
sec  if  it  marks  center  again,  in  which  case,  it  is  moved  slowly 
along  the  entire  length  of  the  straight  edge  to  ascertain  if  it 
is  perfectly  straight  throughout.  The  line  used  for  the  ad- 
justment of  the  dihedral  angle  must  be  well  stretched  to 
prevent  it  from  sagging  and  giving  a  wrong  measurement. 

The  wires  must  be  given  the  proper  tension:  if  they  are 
slack,  the  adjustments  of  the  parts  to  which  they  are  at- 
tached will  be  thrown  out  of  true  when  under  tension;  and 
if  they  are  too  tight,  they  distort  the  parts  and  cause  bending 
stresses,  which  are  the  most  dangerous  in  an  aeroplane 
framework.  For  the  same  reason,  in  handling  a  machine, 
care  must  be  taken  never  to  produce  bending  stresses,  es- 
pecially with  struts.  If  an  aeroplane  is  to  be  moved  about, 
the  points  to  be  taken  hold  of  are  either  the  lower  parts  of 
the  interplane  struts  or  the  upper  parts  of  the  undercarriage 
struts.  Some  machines  have  hand  holes  at  the  wing  tips 
to  facilitate  their  handling  without  damage.  Special  care 
must  be  had  not  to  use  the  trailing  edges  of  the  wings  as  a 
holding  point  to  move  a  machine,  as  they  are  weak  and 
break  easily. 

The  turnbuckles  must  be  well  lubricated,  must  work  freely, 
but  not  loosely,  and  must  be  properly  locked  soon  after  the 
adjustments  of  the  wires  to  prevent  their  slackening. 

All  nuts  must  be  closely  cotter  pinned,  that  is,  the  pins 
must  be  very  close  to  the  nuts  to  avoid  their  loosening. 

The  controlling  planes  deserve  special  consideration  in 
mounting  and  truing  them,  because  they  are  essential  to  the 
safety  of  the  aviator. 


82  AVIATION 

The  engine  rails  must  be  leveled  with  the  greatest  of  care, 
as  they  are  the  basis  of  all  other  adjustments  and  the  slightest 
error  in  them  throws  the  entire  machine  out  of  true,  espe- 
cially at  the  tail  end,  where  the  error  will  be  greatly  in- 
creased. 

The  balance  of  the  machine  depends  on  the  way  it  is  trued 
up.  If,  for  instance,  the  angle  of  incidence  is  smaller  or 
greater  than  it  should  be  or  the  stagger  improperly  adjusted 
or  the  fuselage  distorted  downward  or  upward,  the  longitud- 
inal stability  will  be  affected  and  the  machine  will  fly  tail 
high  or  tail  low.  The  lateral  stability  is  affected  by  setting 
the  wings  at  a  different  angle  of  incidence,  thus  causing  one 
wing  to  fly  low  and  consequently  the  other  high,  owing  to 
their  different  lifting  pcwer.  This  same  fault  causes  the 
machine  to  swing  around,  due  to  the  difference  in  the  drift 
of  the  wings,  thus  unbalancing  the  machine  directionally. 
If  the  fuselage  is  distorted  sideways,  the  machine  has  a 
tendency  to  circle  around,  as,  in  this  case,  it  will  be  offering 
more  keel  surface  on  one  side  than  on  the  other.  If  the  con- 
trolling planes  are  not  set  at  the  proper  angle  or  they  are 
distorted,  the  control  will  be  inefficient.  If  the  angle  of  in- 
cidence of  the  wings  is  greater  than  it  ought  to  be,  besides 
unbalancing  the  machine  longitudinally,  causing  it  to  fly 
nose  high,  it  will  produce  poor  flight  speed,  due  to  the  in- 
creased resistance. 

In  conclusion,  every  error  in  truing  is  felt  by  the  aeroplane, 
and  too  much  emphasis  can  not  be  laid  on  the  fact  that  the 
adjustments  must  be  scrupulously  exact. 


CHAPTER  IV 
PROPELLERS 

Theory. — Propeller  and  mystery  are  synonymous.  In  our 
Year  of  Grace  1919,  nobody  knows  exactly  what  a  propeller 
is.  This  being  the  condition  of  things  at  the  present  day, 
we  can  only  accept  with  the  benefit  of  doubt  whatever  in- 
formation we  can  gather  in  regard  to  propellers. 

The  original  theory  considers  the  propeller  a  section  of  a 
screw  and  therefore  the  blades  portions  of  the  thread;  which 
means  that  the  propeller,  in  revolving,  screws  itself  into  the 
air  and  converts  its  rotary  motion  into  a  linear  motion.  The 
reason  why  only  a  portion  of  the  thread  is  used  is  that  a 
small  slice  of  it  is  found  sufficient  for  propeller  purposes. 
The  number  of  sections  used  represents  the  number  of  blades, 
which  are  made  much  longer  than  the  thread,  because  in 
this  way  they  are  more  efficient. 

The  theory  most  commonly  accepted  to-day  is  based  on 
the  analogy  of  the  propeller  blade  with  a  plane,  the  difference 
being  that  the  plane  moves  in  a  straight  line,  while  the  blade 
moves  in  a  circle,  advancing  in  the  meantime  in  a  straight 
line  and  consequently  describing  a  spiral  path;  in  other 
words,  the  propeller  blade  is  considered  a  revolving  inclined 
plane,  although  even  those  who  accept  this  point  of  view 
admit  that  it  is  not  absolutely  exact,  but  very  useful  as  a 
basis  for  calculation,  whose  results  conform  very  closely  with 
those  obtained  by  experiment. 

The  new  theory  is,  therefore,  a  modification  of  the  old 
one,  substituting  a  plane  for  a  section  of  thread,  but,  ad- 
mittedly, the  facts  deny  the  principle.  The  outcome  is  that 
both  theories  are  found  wanting  and  in  practice  it  is  neces- 
sary to  use  empirical  formulas  to  solve  propeller  problems. 

83    ' 


84  AVIATION 

Another  point  of  view  taken  by  modern  experimenters 
is  the  application  of  the  reflection  or  batting  theory,  that  is, 
the  assumption  that  the  blade  in  striking  the  air  causes  it  to 
jump  off  at  the  same  angle  of  entry  and  the  reaction  imparts 
a  forward  motion  to  the  blade.  Although  this  hypothesis 
is  very  plausible  and  seems  to  be  in  very  close  accord  with 
facts,  it  is  not  sufficiently  developed  to  be  accepted  just  now 
and  we  are  forced,  therefore,  to  follow  the  mixed  principle 
of  the  screw  and  plane. 

The  capital  difference  between  a  screw  working  its  way 
into  a  nut  and  a  propeller  screwing  itself  into  the  air  is  due 
to  the  fact  that  the  air  is  not  solid  like  the  nut  and  the  pro- 
peller blades  can  not  get  as  good  a  grip  on  the  air  as  the 
screw  on  the  nut,  the  result  being  that  the  air  slips  back 
and  the  propeller  can  not  advance  the  full  distance  it  ought 
to  according  to  the  angle  of  the  blades.  This  brings  us  to 
the  consideration  of  three  different  quantities:  the  distance 
the  propeller  ought  to  travel,  the  distance  it  actually  travels 
and  the  distance  lost.  If  we  consider  these  quantities  for 
one  revolution  only,  we  will  have  the  following  definitions: 
theoretical  pitch  is  the  distance  through  which  the  propeller 
would  advance  in  one  revolution  if  it  moved  in  an  unyielding 
medium;  effective  pitch  is  the  distance  actually  traveled  in 
one  revolution;  slip  is  the  difference  between  the  theoretical 
and  the  effective  pitch. 

The  pitch  depends  on  the  angle  of  the  blades  or  angle 
of  pitch. 

If  the  pitch  of  every  point  of  the  blade  of  a  propeller  is 
to  remain  constant,  the  angle  of  pitch  must  increase  as 
the  hub  is  approached,  because  the  nearer  we  get  to  it,  the 
smaller  become  the  diameters  of  the  circles  described  by  the 
propeller  when  revolving  and,  consequently,  the  smaller 
the  circumferences  of  the  circles. 

If  we  mark  the  points  A ,  B,  C  (Fig.  60)  on  one  blade  of  a 
propeller  D  and  then  we  cause  it  to  revolve  on  its  axis  just 
once  without  advancing,  these  points  will  describe  circles, 


PROPELLERS 


85 


which  will  be  smaller  the  nearer  the  point  considered  is  to 
the  hub  E.  From  this,  it  is  clear  that  the  greatest  circle  is 
described  by  the  propeller  tips  and  the  smallest  by  the  center 
of  the  hub,  where  a  circle  is  represented  by  merely  a  point, 


Fig.  60 

consequently  the  smallest  angle  is  at  the  tips  and  the  biggest 
would  be  at  the  center  of  the  propeller,  provided  that  we 
could  build  a  propeller  without  a  hub,  and  even  if  we  could, 
the  angle  would  be  of  no  use  whatever,  as  it  would  be  prac- 
tically a  right  angle. 

To  make  this  clear,  let  us  consider  the  circles  described 
by  the  points  A,  B,  C,  as  the  bases  of  screws  having  all  the 
same  length  (Fig.  61).  If  we  want  these  screws  to  advance 
their  full  length  in  one  revolution  in  their  respective  nuts, 
it  is  evident  that  each  one  must  have  just  one  thread,  starting 


86 


AVIATION 


at  the  top  and  ending  at  the  bottom  of  the  same  side  of  the 
screw.  If  we  wrap  each  screw  with  paper  once  around  with- 
out overlapping  the  ends,  mark 
on  the  paper  the  path  of  the 
thread  and  then  unwrap  and 
flatten  it  out,  we  find  that  the 
thread  is  the  diagonal  of  the 
rectangle,  which  represents  the 
development  of  the  lateral  sur- 
face of  each  screw  (Fig.  62). 
If  we  now  cut  the  rectangles  along  the  diagonals,  and  take 
one-half  of  each  one,  we  will  have  three  right-angled  triangles, 
whose  respective  height  represents  the  distance  advanced  by 


B 

Fig.  61 


LA 

c 


Fig.  62 

each  screw  in  one  revolution  or  the  pitch,  which  is  equal  in 
all  three;  the  bases  represent  the  developed  circumferences 
of  the  circles  of  the  bases  of  the  screws  and  the  hypotenuses 

represent  the  threads. 
The  triangle  thus  formed 
is  the  triangle  of  pitch 
(Fig.  63). 

If  we  put  these  tri- 
angles one  on  top  of  the 
other,  having  the  big- 
gest at  the  bottom  and 
the  smallest  at  the  top, 


Fig.  63 


in  such  a  way  as  to  make  the  heights  coincide  (Fig.  64),  we 
see  that  the  angles  A,  B,  C  are  not  equal,  but  C  is  bigger 
than  B  and  B  bigger  than  A.  As  each  angle  represents  the 


PROPELLERS 


87 


angle  of  pitch  of  each  screw,  we  see  that  the  smaller  the 
screw,  the  bigger  is  the  angle  of  pitch  and  the  steeper  the 
pitch    line    which    the    thread 
must  follow. 

A  small  section  of  each  screw, 
cut  across  its  longitudinal  axis 
(Fig.  65),  will  act  in  the  same 
way  if  screwed  in  its  nut,  be- 
cause the  slice  of  thread  left 
will  work  its  way  through  the 
nut  just  the  same  as  if  the 
thread  were  in  its  entirety. 

We  have  come  to  this  con- 
clusion by  starting  from  the 


Fig.  64 


assumption  that  the  points  A,  B,  C  were  taken  on  the  same 
propeller  and,  consequently,  what  we  have  found  in  the 
case  of  the  screws  of  different  diameters  and  equal  heights 
applies  to  these  points,  and  if  we  curve  around  the  triangles, 
so  that  the  bases  form  circles,  and  we  put  them  one  inside 
the  other  (Fig.  66),  the  hypotenuses  indicate  the  pitch  lines 


Fig.  65 

of  the  points  A,  B,  C,  that  is,  the  spiral  paths  which  they 
would  follow  respectively  if  the  propeller  advanced  through 
the  air.  In  other  words,  we  may  consider  a  screw  propeller 
as  composed  of  an  immense  number  of  screws  one  inside  the 
other,  out  of  which  all  the  unnecessary  parts  have  been  cut 
out,  leaving  only  the  center  screw  as  a  hub  and  attaching 
to  its  thread  the  threads  of  all  the  following  screws.  This 
would  give  us  only  one  blade,  which,  of  course,  can  be  re- 
produced, giving  us  the  means  of  making  a  propeller  with 
as  many  blades  as  desired;  but,  following  the  analogy  of 


AVIATION 


the  propeller  blade  with  the  plane,  the  same  law  of  inter- 
ference which  limits  the  gap  holds  true,  but  only  in  so  far 
as  it  is  applicable  to  the  propeller. 

In  the  case  of  superposed  planes,  we  are  free  to  make  the 
gap  the  distance  required  or  to  stagger  the  planes,  but  with 

a  propeller  this  is  not 
possible,  as  the  blades 
are  fixed  both  in  po- 
sition and  distance, 
and  the  only  thing 
left  is  to  make  the 
width  of  the  blade 
proportional  to  the 
gap  or  vertical  dis- 
tance between  two 


Fig.  66 


consecutive  helicoid- 
al  paths  in  order  to 
regulate  the  amount  of  compression  and  rarefaction  caused 
by  the  camber  of  the  propeller  and  avoid  interference.  This 
means  that  the  greater  the  number  of  blades  of  a  screw  pro- 
peller, the  smaller  must  be  the  width  of  the  blades  and, 
consequently,  the  greater  their  aspect  ratio. 

The  consideration  of  aspect  ratio  brings  us  to  that  of  the 
diameter  of  the  propeller,  which  is  of  the  greatest  importance, 
because  a  propeller  of  large  diameter  is  more  efficient  in  the 
utilization  of  power  for  several  reasons.  The  thrust  or 
power  delivered  by  the  propeller  is  concentrated  on  its  blades 
and,  therefore,  the  larger  the  area  on  which  the  thrust  is 
distributed,  the  smaller  its  proportion  for  unit  surface  and 
the  more  able  is  the  propeller  to  withstand  the  stress  with- 
out breaking.  To  obtain  the  greatest  efficiency,  all  parts 
of  the  blade  should  do  the  same  amount  of  work,  but  as 
the  angle  of  pitch  increases  towards  the  hub,  the  nearer  we 
get  to  it,  the  smaller  will  be  the  thrust  drift  ratio,  the  greater 
the  disturbance  of  the  wash  caused  by  the  hub  and  the 
smaller  the  efficiency,  so  that  the  greatest  quantity  of  work, 


PROPELLERS  89 

if  not  all  of  it,  is  done  by  about  two-thirds  of  the  outer  part 
of  the  blade,  the  inner  part  being  designed  for  low  resistance 
rather  than  for  driving.  An  increase  in  diameter  really 
means  an  increase  in  the  efficient  part  of  the  blade,  as  the 
further  out  we  go,  the  smaller  we  find  the  angle  of  pitch 
and  the  greater  the  thrust  drift  ratio  and,  consequently,  the 
greater  the  efficiency.  The  efficiency  of  a  propeller  increases 
with  the  increase  in  diameter,  because  the  area  swept  by 
the  propeller  increases  with  the  square  of  the  diameter, 
which  results  in  a  reduction  of  slip:  the  greater  the  diameter, 
the  lower  the  speed  necessary  to  run  the  propeller  and  in 
consequence  of  both  the  increased  surface  and  the  dimin- 
ished speed,  the  propeller  gets  a  better  hold  on  the  air  and 
does  more  useful  work,  reducing  to  a  minimum  the  wasteful 
slip. 

While  we  can  safely  say  that  a  screw  propeller  must  have 
a  large  diameter  to  be  more  efficient,  we  must  consider,  on 
the  other  hand,  the  limit  imposed  by  the  strength  and  weight 
of  the  material. 

Another  very  important  consideration  is  the  proportion 
between  the  pitch  and  the  diameter  of  a  propeller  or  pitch 
ratio,  which  varies  for  different  cases  of  service,  a  high-speed 
machine  requiring  a  higher  pitch  ratio  than  a  low-speed 
machine.  To  obtain  the  best  efficiency,  the  pitch  must  be 
about  1}4  times  the  diameter,  but  when  we  take  into  con- 
sideration the  diameter  of  the  propeller,  which  must  be  as 
large  as  possible  to  be  more  efficient,  the  high  speed  of  the 
gasoline  motor  and  the  relatively  slow  speed  of  the  majority 
of  the  machines  used  to-day,  we  see  that  it  is  not  always 
possible  to  obtain  the  pitch  ratio  of  best  efficiency,  and  we 
find  that  the  propeller  used  at  present  for  aeroplanes  is  of 
finer  pitch  than  that  of  best  efficiency. 

In  the  case  of  a  machine  whose  flight  speed  is  50  M.  P.  H., 
with  a  motor  running  at  the  speed  of  1100  R.  P.  M.  and 
an  8-foot  diameter  propeller,  we  find  that  the  effective  pitch 
must  be  4  feet,  which  is  just  one-half  the  diameter  of  the 


90  AVIATION 

propeller,  instead  of  10  feet,  as  it  ought  to  be  for  best  effi- 
ciency. If  the  flight  speed  of  the  machine  were  instead  125 
miles  per  hour,  then  the  effective  pitch  would  be  10  feet  and 
the  requirement  of  best  pitch  ratio  fulfilled.  From  this, 
we  see  that  as  the  speed  of  the  machine  increases,  the  in- 
compatibility between  the  speed  of  the  motor  and  the  pitch 
ratio  decreases,  and  in  the  future,  when  all  machines  will 
have  reached  the  highest  speed,  this  incompatibility  will 
be  entirely  eliminated.  At  the  present,  this  could  be  ac- 
complished by  gearing  down  from  the  engine  to  the  pro- 
peller, but  as  this  would  cause  a  loss  of  about  4  per  cent 
of  the  power  and  as  the  loss  of  efficiency  with  the  direct 
coupled  propeller  is  not  great,  the  direct  drive  is  preferred, 
especially  as  it  produces  a  lower  torque  on  the  crank  shaft 
and  a  consequent  lower  effect  of  the  propeller  torque  on  the 
machine. 

A  screw  propeller  is  usually  designed  for  the  greatest 
velocity  of  an  aeroplane,  so  that  for  a  diminution  in  speed, 
the  efficiency  of  the  propeller  diminishes  also. 

In  laying  out  a  constant  pitch  propeller  blade,  the  first 
consideration  is  the  determination  of  the  blade  angles  at 
different  radii  as  modified  by  the  slip;  then  the  centers  of 
figure  of  the  sections,  which  should  all  coincide  with  the 
axis  of  the  blade;  and,  finally,  the  centers  of  pressure  of 
the  different  sections,  which  should  be  so  disposed  as  to 
avoid  any  twisting  effect  on  the  blade. 

A  matter  of  controversial  nature  is  the  existence  of  cavita- 
tion  in  an  aeronautical  propeller.  Cavitation  would  be  the 
rarefaction  of  air  produced  in  the  space  immediately  in 
the  rear  of  a  swiftly  revolving  propeller  blade,  due  to  the 
rapid  cleavage  of  the  air  by  the  blade  and  the  relatively 
slow  action  of  the  air  in  closing  in  behind  the  moving  blade, 
and  while  it  is  generally  said  to  make  its  appearance  at  about 
1500  R.  P.  M.,  there  are  those  who  flatly  reject  the  theory 
in  the  case  of  aeronautical  propellers.  Cavitation  is  ad- 
mitted by  all  to  exist  in  marine  propellers,  and  this,  coupled 


PROPELLERS  91 

with  the  fact  that  the  aspect  ratio  of  a  marine  propeller 
blade  is  limited  by  the  much  greater  pressure  reaction  due 
to  the  much  greater  density  of  the  water  as  compared  to 
the  air,  explains  the  lower  efficiency  of  a  marine  propeller, 
which  at  most  is  about  75  per  cent,  while  for  aeronautical 
propellers  is  about  85  per  cent.  No  material  known  to-day 
would  stand  the  increased  pressure  due  to  an  attempt  at 
increasing  the  efficiency  of  a  marine  propeller,  while  for  air 
propellers  the  use  of  the  softest  wood  would  compare  far 
better. 

In  regard  to  the  position  of  the  propeller  in  front  or  in 
the  rear  of  an  aeroplane,  there  are  advantages  and  disad- 
vantages in  either  case. 

The  power  used  to  drive  the  machine  forward  is  spent  in 
imparting  a  forward  motion  to  the  air,  so,  if  we  place  the 
propeller  at  the  rear,  it  will  be  running  in  air  which  is  al- 
ready moving  forward  at  a  great  rate  of  speed  and  this  has 
the  effect  of  greatly  reducing  the  slip,  and  if  the  slip  should 
be  equal  to  the  forward  motion  of  the  air,  then  the  apparent 
slip,  that  is,  the  difference  between  the  velocity  of  the  pro- 
peller and  the  velocity  of  the  machine,  would  be  zero  and 
the  machine  would  be  flying  just  as  fast  as  if  the  propeller 
were  screwing  its  way  through  a  solid  medium.  It  may 
happen,  and  it  has  already  actually  happened  with  steam- 
ers, that  the  forward  motion  of  the  fluid  in  which  the 
machine  is  running  is  greater  than  the  slip,  and  in  this 
abnormal  case,  the  machine  would  be  going  faster  than 
if  the  propeller  were  running  in  a  solid  nut.  This  is 
the  case  of  negative  slip,  that  is,  the  velocity  of  the  ma- 
chine is  greater  than  the  velocity  of  the  propeller.  But 
while  these  are  merely  theoretical  considerations,  some 
of  which  may  or  may  not  materialize,  the  fact  remains,  as 
we  have  already  seen,  that  a  propeller  in  the  rear  means  a 
specially  constructed  and  clumsy  fuselage,  with  attendant 
outriggers,  struts  and  wires,  which  increase  the  weight  and 
the  drift  of  the  machine,  probably  eliminating  altogether 


92 


AVIATION 


the  advantages  of  having  the  propeller  in  the  rear.  When 
the  propeller  is  in  front,  the  fuselage  can  be  built  in  the  best 
stream-lined  shape,  which  reduces  the  resistance  and  weight 
to  a  minimum,  and  besides,  due  to  the  fact  that  the  air  is 
blown  against  the  wings,  the  lift  is  more  than  doubled,  both 
because  the  speed  of  the  machine  relatively  to  the  air  is 
increased  and  because  more  air  is  engaged,  and  we  know 
that  the  lift  is  proportional  to  the  amount  of  air  engaged 
and  to  the  square  of  the  velocity.  While  this  is  a  great  ad- 
vantage, it  is  in  the  meantime  a  great  disadvantage,  be- 
cause the  air  blown  against  the  machine  has  also  the  effect 
of  pushing  it  backward,  so  that  the  effective  forward  motion 
is  the  difference  between  the  two  forces;  and  although  the 
increased  lift  enables  us  to  reduce  the  span,  on  the  other 
hand,  the  increase  in  passive  drift  requires  the  employment 
of  a  considerably  greater  horse  power. 

Problems. — To  find  the  angle  of  pitch  at  a  given  point  of 

a  constant  pitch  pro- 
peller, it  is  necessary 
to  know  the  pitch,  and 
vice  versa;  that  is,  to 
find  the  pitch,  we  must 
know  the  angle. 

If  the  pitch  of  a  pro- 
peller A  (Fig.  67a)  is 
6  feet  and  we  want  to 
find  the  angle  of  pitch 


Fig.  67 


at  a  given  point  B,  3  feet  out  from  the  center,  we  can  solve 
the  problem  graphically  by  means  of  the  triangle  of  pitch. 
First  we  find  the  circumference  described  by  the  given 
point  B  in  one  revolution  by  multiplying  the  radius  by  2 
to  find  the  diameter  and  then  by  3.14;  that  is,  3  x  2  x  3.14  = 
18.84.  We  mark  this  distance  on  a  line  C  D  (Fig.  676) ;  from 
one  of  its  ends  D  draw  the  perpendicular  D  E,  equal  to  the 
pitch,  and  from  the  other  end  C,  a  line  C  E  which  joins  the 
extreme  points  of  the  developed  circumference  and  the 


PROPELLERS  93 

pitch.  The  angle  a,  formed  by  the  base  and  the  hypotenuse 
of  the  triangle,  is  the  angle  of  pitch  at  the  given  point  B  of 
the  propeller. 

If,  instead,  the  angle  a  is  known,  we  find  the  circumference 
C  D,  lay  the  angle  on  one  of  its  ends  C,  from  this  point  draw 
an  indefinite  line  C  F,  inclined  at  an  angle  equal  to  that 
given  a,  and  from  the  other  end  D  the  perpendicular  D  E. 
The  point  of  intersection  E  of  the  two  lines  determines  the 
pitch  D  E. 

If  we  want  to  find  the  pitch  of  a  propeller  in  a  numerical 
way,  given  the  machine  speed,  the  motor  speed  and  the  pro- 
peller slip,  the  problem  is  solved  in  the  following  way  : 

If  the  data  are  these:  machine  speed,  50  M.  P.  H.;  motor 
speed,  1100  R.  P.  M.;  and  the  propeller  slip,  20  per  cent; 
we  reduce  first  the  miles  per  hour  to  feet  per  hour  by  multi- 
plying 50  by  5280,  then  we  divide  this  product  by  60  to 
find  the  feet  per  minute,  and  finally  we  divide  this  quotient 
by  1100  to  find  the  number  of  feet  in  one  revolution.  This 
last  result  tells  us  what  the  effective  pitch  of  the  propeller 
should  be  to  get  the  speed  of  50  M.  P.  H.,  and  as  the  slip  is 
20  per  cent,  we  must  increase  accordingly  the  number  found 
to  obtain  the  theoretical  pitch  of  the  propeller,  so  that  when 
the  slip  is  deducted,  we  actually  get  the  necessary  effective 
pitch. 

We  have  then: 


50x5280=264000,  =4400,          = 

DU 


If  the  slip  is  20  per  cent,  the  efficiency  of  the  propeller  is 
80  per  cent,  and  as  4  represents  this  80  per  cent,  we  can  get 
the  theoretical  pitch  from  the  following  proportion: 


The  pitch  of  the  propeller  is,  therefore,  5  feet. 
Manufacture.  —  Propellers  may  be  made  of  metal  or  wood. 


94  AVIATION 

Metal  propellers  have  the  advantage  of  cheapness  as  com- 
pared with  wooden  propellers,  but  they  have,  on  the  other 
hand,  certain  drawbacks,  which  give  rise  to  objection  to  their 
use.  First  of  all  they  are  heavy,  and  if  they  should  burst 
under  the  strain  of  high  velocity,  the  fragments  are  apt  to 
cause  damage.  Then  they  bend  easily,  and  on  account  of 
their  great  elasticity  they  vibrate  when  in  use.  Another 
drawback  is  the  quasi  impossibility  of  obtaining  an  even  sur- 
face blade,  and  finally  the  difficulty  of  attaching  the  blades 
to  the  propeller  arm. 

If  a  metal  propeller  is  to  be  used,  perhaps  aluminum  is  the 
more  suitable  to  make  the  surfaces  of  the  blades,  because  its 
lightness  permits  of  relatively  thick  blades,  which,  increasing 
the  moment  of  inertia,  preserve  their  shape.  But,  all  things 
considered,  wood  propellers  are  the  best,  even  if  they  cost 
more. 

Wood  propellers  are  light,  and  this  is  their  chief  charac- 
teristic, from  which  many  a  good  advantage  is  derived. 
Being  light,  they  can  be  made  very  thick.  Their  thickness 
makes  it  possible  to  shape  the  blades  in  a  way  to  offer  the 
least  resistance  to  motion,  and  again  to  cause  an  increase 
in  the  moment  of  inertia,  with  a  consequent  increase  in  the 
resistance  to  flexure,  which  permits,  therefore,  of  a  very 
high  rate  of  speed  with  very  little  probability  of  bursting, 
as  wood  possesses  greater  tensile  strength  than  the  best 
metal,  especially  with  the  grain  running  in  the  sense  of  the 
length  of  the  blade;  but  even  if  the  propeller  should  burst, 
the  fragments  being  light  would  not  be  so  dangerous. 

Wood  propellers  can  be  made  in  one  single  piece  or  in 
laminations,  which  are  glued  together  with  insoluble  glue. 
One-piece  propellers  are  cheaper,  but  as  it  is  hard  to  find 
wood  of  straight  grain  without  any  flaws,  the  laminated  pro- 
pellers are  to  be  preferred,  because  they  allow  the  use  of  the 
best  kind  of  wood. 

A  propeller  is  usually  made  with  five  or  six  laminations  of 
mahogany,  walnut  or  oak.  The  laminations,  besides  being 


PROPELLERS  95 

glued  together,  are  held  in  place  by  dowels  driven  through 
them  at  equal  distances.  They  are  held  in  a  press  until 
thoroughly  dry,  then  they  are  cut  by  machinery  into  pro- 
peller shape  and  finished  by  hand. 

The  wood  used  for  a  propeller  must  not  be  very  dry,  but 
it  must  contain  a  given  amount  of  moisture,  which  must 
be  kept  constant,  and  to  this  effect  the  propeller  is  painted 
with  a  special  filler  and  then  varnished  several  times. 

Some  propellers  have  metallic  protections  at  the  tips  and 


Fig.  68 

as  the  metal  can  not  adhere  to  the  wood,  if  the  machine 
were  in  flight  on  a  rainy  day,  the  rain  would  filter  through 
and  collect  at  the  tip,  causing  the  metal  to  bulge  up  and 
tear  the  propeller  to  pieces  owing  to  the  terrific  centrifugal 
force.  To  avoid  this,  small  holes  are  bored  at  the  tips  of 
the  metallic  protections  to  allow  the  water  to  run  out. 

Balance. — A  propeller  must  be  perfectly  balanced.  There 
are  different  methods  to  try  a  propeller  for  balance,  but  the 
best  is  to  mark  equal  distances  from  the  center  to  the  tip 
on  both  blades  and  to  weigh  the  propeller  at  all  the  marks; 
for  an  equal  distance  from  the  center,  the  weight  on  one 
blade  must  be  equal  to  that  on  the  other.  This  is  accom- 
plished by  inserting  one  of  the  tips  D  (Fig.  68)  in  a  notch 
cut  in  a  wall  and  hooking  the  propeller  to  a  spring  scale  E 


96 


AVIATION 


suspended  from  a  bar  F  and  weighing  it  at  the  different 
equidistant  marks  A,  B,  C  and  A',  B',  C".  The  weight  at 
the  point  of  the  first  mark  A  on  one  side  of  the  propeller 
must  be  equal  to  that  of  the  first  mark  A'  on  the  other  side; 
the  second  equal  to  the  second,  and  so  on. 

A  slight  error  in  the  balance  can  be  corrected  by  additional 
coatings  of  varnish  on  the  blade  which  weighs  less  or  by 
scraping  off  some  of  the  material  near  the  hub  of  the  blade 
which  is  heavier.  If  the  difference  is  too  much  and  can  not 
be  corrected  in  this  way,  the  propeller  must  be  rejected. 

Test. — To  see  if  the  pitch  angle  of  a  propeller  is  correct, 
it  is  measured  on  both  blades  at  equal  distances  from  the 

center  by  means  of  a 
protractor  or  the  tri- 
angle of  pitch.  The 
angles  equidistant 
from  the  center  must 
be  equal.  To  accept 
a  propeller  as  good, 
the  pitch  angle  must 
be  within  Y^  a  de- 
gree of  the  proper 
angle. 

To  test  a  propeller  for  warpage,  the  following  method 
may  be  used : 

Starting  from  the  center  A  (Fig.  69a)  of  a  propeller  B, 
we  mark  different  points  C,  D  and  E  on  one  blade;  then, 
by  means  of  a  protractor,  we  measure  the  angle  a  at  the 
inner  point  C  and  lay  it  off  at  one  end  A  (Fig.  696)  of  the 
line  A  B,  which  represents  the  developed  circumference  of 
the  circle  described  by  the  given  point;  from  the  same  end  A, 
we  draw  an  indefinite  line  A  C,  inclined  at  an  angle  a  equal  to 
that  measured,  and  from  the  other  end  B,  the  perpendicular 
B  C,  which  is  the  pitch  of  the  propeller.  The  same  process 
is  repeated  for  all  the  other  points  and  if  the  propeller  is  not 
warped,  the  lines  which  represent  the  pitch  should  all  be 


PROPELLERS  97 

equal,  B  C  =  D  E=F  G.  To  facilitate  the  work,  we  draw 
the  parallel  C  G  from  the  point  of  intersection  C  of  the  first 
pitch  found  to  the  line  A  B,  which  represents  the  developed 
circumference  of  the  first  point,  and  if  the  blade  is  correct, 
all  the  other  points  fall  exactly  on  this  parallel  line;  but  if 
they  fall  within  or  without  it,  the  angles  are  smaller  or 
larger  than  they  ought  to  be  and  the  blade  is  warped.  If, 
instead,  it  is  not  warped,  the  same  system  is  followed  to  test 
the  other  blade. 

The  length  of  one  blade  must  be  equal  to  that  of  the  other 
or  the  difference  must  fall  within  1/16  of  an  inch  to  accept 
the  propeller  as  good. 

The  width  and  the  camber  of  the  blades  must  be  equal  at 
points  equally  distant  from  the  center. 

An  error  of  1/8  of  an  inch  is  allowed  for  the  straightness 
of  a  propeller. 

The  joints  of  the  laminations  must  be  all  perfectly  closed 
and  the  surface  very  smooth  throughout. 

The  hub  hole  and  the  bolt  holes  must  be  perfectly  straight 
and  at  right  angles  with  the  face  of  the  hub. 

Care. — A  propeller  must  be  kept  always  in  a  vertical 
position  to  protect  it  from  distortion,  and  the  best  way  is 
to  mount  it  on  a  wooden  peg,  which  fits  the  hub  hole  exactly. 

To  prevent  the  blades  from  warping,  the  place  where  the 
propeller  is  stored  must  be  neither  very  damp  or  very  dry, 
nor  such  as  to  allow  the  sun  rays  to  fall  on  it.  If  these  rules 
are  disregarded,  the  propeller  will  lose  its  efficiency  and  give 
rise  to  flutter,  which  will  stress  the  bearings  and  crank  shaft 
of  the  motor  and  probably  tear  it  to  pieces. 

Boss. — Boss  is  a  metallic  device  used  for  the  attachment 
of  the  propeller  to  the  shaft  of  the  motor. 

The  simplest  form  of  boss  consists  of  two  flanges,  one  of 
which,  A  (Fig.  70),  is  permanently  attached  to  the  shaft  and 
has  eight  threaded  holes,  while  the  other  B  is  a  separate 
piece  with  plain  holes.  The  propeller  C  is  mounted  on  the 
shaft  between  the  flanges,  which  are  held  together  by  eight 


98 


AVIATION 


bolts  on  whose  points  are  also  screwed  and  cotter  pinned  the 
nuts.    While  this  is  the  simplest  form  of  boss,  it  is  not  the 
C  best,  because  the  bolts  are  shaken  and 

loosened  by   the  revolution  of  the  pro- 
peller, which  is  liable  to  break. 

A  good  form  of  propeller  boss  is  that 
used  for  the  Gnome  motor  (Fig.  71).  In 
this  case,  the  flanges  are  also  two  A 
and  B,  but  they  are  both  attached  to  the 
propeller  by  eight  bolts,  and  one  of  the 
flanges  A  has  a  tubular  projection  C  with 
a  keyway  Z>,  cut  in  the  inside,  in  which, 
when  the  propeller  is  mounted,  fits  a  key 
E  laid  in  a  slot  F  cut  in  the  shaft,  and  the  propeller  is  then 
held  in  place  by  a  nut  G  screwed  on  the  crank  shaft.  To 
prevent  this  nut  from  unscrewing,  a  ring  spring  H  is  mounted 
on  it  in  such  a  way  that  one  of  its  points  /  goes  through  one 
of  three  holes  J  bored  in  the  nut  and  inside  one  of  four 
slots  K  cut  on  the  end  of  the  crank  shaft. 


Fig.  70 


Fig.  71 

The  Curtiss  boss  (Fig.  72)  is  similar  to  the  Gnome,  with 
the  difference  that,  beside  the  eight  bolts,  there  is  a  nut  A 
screwed  on  the  threaded  end  of  the  tubular  projection  to 
help  fasten  the  flanges  on  the  propeller,  which  is  then  held 


PROPELLERS 


99 


Fig.  72 


in  place  on  the  shaft  by  screwing  on  it  another  nut  B.  Both 
nuts  have  three  holes  for  the  use  of  springs,  which  lock  them 
in  the  slots  cut  in  the  tubular  pro- 
jection and  in  the  shalt,  as  in  the 
Gnome  boss. 

A  propeller  must  be  mounted  at 
right  angles  with  the  shaft.  To  test 
the  alignment,  a  stick  is  brought  in 
contact  with  the  tip  of  one  blade  and 
held  in  position  while  the  other  blade 
is  brought  around  to  see  if  it  touches  the  point  of  the  stick 
as  the  first  blade.  If  this  is  not  the  case,  the  nuts  must  be 
tightened  on  the  side  of  the  blade  which  forms  the  greater 
angle  with  the  shaft,  until  the  propeller  is  perfectly  aligned. 

As  in  an  aeroplane  the  motor  is  cranked  by  means  of  the 
propeller,  another  point  to  consider  in  mounting  it  is  to 
put  it  in  such  a  position  that  it  can  be  grasped  easily  and 
the  motor  started  quickly.  This  means  that  the  propeller 
must  be  in  an  inclined  position  and  one  of  the  cylinders  of 
the  motor  on  compression,  near  the  firing  point.  If  this  is 
not  done,  it  will  be  very  dangerous  and  hard,  if  not  impos- 
sible, to  start  the  motor. 


CHAPTER  V 
MAINTENANCE 

Inspection. — With  the  exception  of  the  medical  profession, 
there  is  perhaps  no  other  calling  in  life  which  has  more  re- 
sponsibility than  that  of  the  aeroplane  mechanic:  to  him  is 
entrusted  the  fate  of  human  beings,  who  may  be  dashed  to 
instant  death  through  a  mere  carelessness  on  his  part,  and 
there  can  be  no  greater  remorse  than  that  caused  by  the 
remembrance  of  having  destroyed  human  life  through  lack 
of  care. 

To  avoid  these  dire  consequences,  it  is  imperative  that  an 
aeroplane  be  scrupulously  inspected  before  and  after  every 
flight,  daily  and  weekly,  and  maintained  in  the  best  condition. 

To  inspect  a  machine  before  or  -after  a  flight,  the  most 
important  parts  are  looked  after,  such  as  the  bracing  and 
control  wires,  to  see  if  they  are  properly  tensioned  and  in 
good  condition.  The  machine  must  also  be  cleaned  from  the 
dust  that  collects  on  the  wires,  which  are  oiled  or  greased, 
on  the  planes  and  on  all  the  other  parts  in  general. .  When 
the  machine  starts  from  or  lands  on  wet  ground,  the  wheels 
throw  off  mud  on  the  under  side  of  the  wings  and  it  must 
be  removed,  which  is  easy  to  do  if  the  mud  is  wet,  but  if 
dry,  it  must  first  be  dampened,  otherwise  the  fabric  may  be 
damaged  in  scraping  it  off.  The  motor  also  throws  oil  all 
over  the  machine.  If  any  of  it  goes  on  the  planes,  it  must 
be  cleaned  with  gasoline,  acetone  or  hot  water  and  soap. 
When  soap  is  used,  it  must  be  of  the  kind  that  has  no  alkali, 
that  is,  no  soda  or  potash,  which  damages  the  fabric. 

In  a  daily  inspection,  besides  what  is  done  for  a  flight  in- 
spection, all  the  adjustments  of  the  machine  must  be  looked 
after,  to  see  if  the  straightness  of  the  wings,  the  dihedral 

100 


i '-',  i 101 

angle,  the  angle  of  incidence,  the  stagger  and  the  controls 
are  in  perfect  order. 

In  a  weekly  inspection,  all  the  parts  of  the  machine,  from 
the  biggest  to  the  smallest,  must  be  carefully  examined,  and 
the  best  way  to  do  this  is  to  have  an  inspection  card  with 
all  their  names  written  down,  to  check  them  off,  one  after 
the  other,  as  they  are  examined. 

The  wires  are  first  inspected  to  see  that  they  are  not 
damaged,  scored,  kinked  or  rusty,  and  then  they  are  greased 
or  oiled. 

The  control  cables  must  be  inspected  thoroughly,  espe- 
cially around  the  pulleys,  where  the  wires  are  apt  to  fray. 
If  even  one  only  of  the  small  wires  is  broken,  the  cable  must 
be  replaced.  As  these  wires  are  covered  with  grease,  they 
must  be  washed  with  gasoline  to  facilitate  the  inspection. 
The  control  cable  connections  must  also  be  examined  to 
see  if  they  are  in  perfect  order. 

The  fittings  must  be  looked  after  for  signs  of  cracking  at 
the  corners. 

All  the  locking  arrangements,  that  is,  the  safety  wires  of 
the  turnbuckles,  the  cotter  pins  and  the  nuts,  must  all  be 
properly  set  in  place. 

The  axle  and  spokes  of  the  wheels,  the  rudder  post,  the 
tail  skid  post  and  the  struts  must  be  examined  for  distorsion 
and  replaced  if  necessary. 

All  moving  parts,  that  is,  wheels,  pulleys,  hinges  and 
control  mechanism  must  be  well  lubricated  with  graphite. 

The  shock  absorbers  of  the  wheels  and  tail  skid  must 
be  thoroughly  inspected,  to  see  if  they  have  the  proper  ten- 
sion and  if  they  show  any  signs  of  wear. 

The  fabric  must  be  examined  for  wear  and  tear,  and  for 
the  condition  of  the  dope  and  varnish. 

It  is  a  good  practice  to  stand  some  distance  away  from 
the  machine  every  time  it  is  adjusted,  to  get  used  to  the 
way  it  looks  and  learn  to  see  at  a  glance  its  condition,  thus 
saving  time  in  the  inspection.  If,  for  instance,  we  stand  in 


102  AVIATION 

front  of  the  machine  and  look  at  the  front  struts,  when 
they  are  properly  aligned,  they  must  cover  the  rear  ones, 
and  if  we  look  at  them  from  the  side,  the  outer  struts  must 
be  in  a  line  with  the  inner  struts.  The  straightness  of  the 
wings,  both  in  regard  to  the  leading  and  trailing  edges,  can 
be  easily  detected  by  looking  at  them  from  the  front  or  rear 
of  the  machine. 

It  is  well  to  time  every  inspection,  either  partial  or  gen- 
eral, to  know  exactly  how  long  it  takes  and  be  ready  for 
any  emergency. 

Forced  Landing. — In  case  of  a  forced  landing  in  a  cross 
country  flight,  the  first  thing  to  do  is  to  choose  the  proper 
ground  to  start  from  at  any  time,  because  the  weather  may 
change  suddenly  and  if  the  proper  spot  is  not  chosen  before- 
hand, the  start  can  not  be  made  immediately;  then  the 
machine  must  be  turned  around  to  face  the  wind  and,  if 
possible,  put  under  shelter.  It  is  a  good  plan  to  dig  trenches 
and  sink  the  wheels  in  them  or,  if  this  is  not  .possible,  to 
block  the  wheels  to  prevent  the  wind  from  blowing  the 
machine  away. 

If  the  machine  is  to  remain  exposed  any  length  of  time 
on  a  windy  day,  it  is  well  to  picket  it  by  tying  ropes  from  the 
lower  part  of  the  interplane  struts  or  the  upper  part  of  the 
undercarriage  struts  to  pickets  driven  in  the  ground.  In 
this  case,  all  points  where  the  cord  comes  in  contact  with  the 
fabric  must  be  padded  with  soft  material,  otherwise  the 
rubbing  of  the  cord  spoils  it. 

The  controls  must  be  lashed  fast  to  avoid  damage  to  them 
by  being  blown  about  by  the  wind. 

If  the  machine  is  to  stay  in  the  open  overnight,  the  pro- 
peller must  be  covered  to  protect  it  from  moisture. 

Repairs. — Wood. — The  repairs  of  the  wooden  parts  of  an 
aeroplane  usually  consist  in  replacing  broken  members  with 
new  ones,  unless  it  is  an  emergency  repair  in  an  exceptional 
case,  when  the  highest  skill  and  attention  are  necessary  to 
prevent  the  collapsing  of  the  part  temporarily  fixed. 


MAINTENANCE  103 

In  substituting  a  new  piece,  care  should  be  taken  to  see 
that  it  fit  exactly,  and  if  there  are  any  holes  for  the  passage 
of  bolts,  that  they  be  the  proper  size.  If  the  holes  are  larger 
than  they  ought  to  be,  the  bolts  move  and  throw  the  parts 
fixed  out  of  adjustment,  and  if  they  are  smaller,  the  intro- 
duction of  the  bolts  may  split  the  wood. 

The  washers  used  for  wood  must  be  larger  than  those 
for  metal  to  give  them  a  greater  supporting  surface  and 
prevent  their  sinking  into  the  wood. 

Metal. — The  fittings  are  generally  made  with  several 
strips  of  pressed  steel,  cut  in  the  proper  shape,  riveted  to- 
gether, brazed  and  coated  with  non-rusting  paint.  If  this 
process  is  not  available  in  making  a  fitting,  the  best  thing 
is  to  make  it  in  one  piece,  cutting  the  plate  accordingly. 
In  bending  the  plate,  care  must  be  taken  not  to  form  sharp 
corners,  as  they  weaken  the  metal  and  cause  it  to  break,  and 
as  pressed  steel  has  a  grain  running  in  one  direction  only,  the 
bends  must  be  made  across  the  grain,  otherwise  they  are  weak. 

In  substituting  an  old  wire  with  a  new  one,  it  is  essential 
to  use  the  proper  quality,  and  to  test  it,  the  easiest  way  is  to 
lock  in  a  vise  a  short  piece  of  it  and  bend  it  at  a  right  angle. 
If  the  wire  flattens  at  the  curve,  it  is  too  soft ;  if  it  roughens, 
that  is,  shows  small  cracks  at  the  outside  of  the  bend,  it  is 
too  hard;  and  if  it  does  not  show  any  of  these  two  signs,  it  is 
the  right  kind  to  use. 

The  solid  wires  inside  of  planes  must  be  coated  with  white 
non-rusting  paint,  and  not  with  red  paint,  because  any  signs 
of  rust  do  not  show  when  covered  with  red,  while  they  do 
through  white,  and  give  a  warning.  This  white  paint  is  also 
more  elastic  than  the  red  and  does  not  crack  with  changes 
in  temperature.  The  wires  must  be  dry  before  being  painted, 
otherwise  the  paint  does  not  adhere,  peels  off  in  due  time 
and  exposes  the  wires,  thus  failing  to  protect  them  from 
dampness. 

As  all  wires  must  be  looped  to  be  used,  it  is  necessary  to 
know  how  these  loops  are  made. 


104 


AVIATION 


Fig.  73 


To  make  the  ferrule  and  loop,  the  solid  wire  is  inserted  in 
a  ferrule  A  (Fig.  73a)  and  in  a  shank  B,  the  wire,  is  looped, 
^~^  the   shank   slid   in   the   loop, 

i       r-^      J--'1  the   ferrule   slipped   back   to 

J\\  rest    against    the    loop,    the 

CL,  ¥  short  length  of  the  wire  bent 

B  over  the  ferrule  and  then  cut 

off,  leaving  just  a  small  hook 
to  hold  the  ferrule  in  place 
(Fig.  736). 

In  this  process,  the  follow- 
ing points  must  be  observed:  in  making  the  loop,  a  good 
length  of  wire  must  be  allowed  to  be  easily  bent  before  cut- 
ting, as  solid  wire  is  stiff  and  can  not  be  bent  if  it  is  short; 
and  the  loop  must  be  oval,  well  defined,  symmetrical,  with- 
out scores  or  angular  corners  to  weaken  the  wire  and  cause 
it  to  break,  and  of  small 
size,  otherwise  it  elongates 
easily  under  tension  and 
throws  out  of  adjustment 
the  parts  to  which  it  is 
connected. 

If  no  ferrule  is  available, 
one  may  be  made  by  cut- 
ting a  short  copper  tube  of 
suitable  size  and  flattening 
it  out  just  enough  to  make 
it  oval. 

The  ferrule  and  loop  is 
often  dipped  in  solder  to 
fill  in  the  space  between 
the  ferrule  and  the  wire. 

A  spliced  loop  is  made  by  unwinding  the  strands  of  a 
wire  A  (Fig.  74a),  making  the  loop  around  a  thimble  B 
(Fig.  746)  and  inserting  one  strand  at  a  time  in  its  closed 
strarids  C  by  prying  them  open  with  a  pointed  tool.  After 


Fig.  74 


MAINTENANCE 


105 


some  length  has  thus  been  spliced,  one  wire  of  each  strand 
is  cut  off  and  the  splicing  continued;  when  another  length 
is  spliced,  another  wire  is  cut  off,  and  so  on,  until  the  loop 
is  finished.  The  wires  are  cut  off  gradually  to  taper  the 
splicing  down  towards  the  end.  The  splice  is  served  with 
a  fine  string  or  a  wire  D  (Fig.  74c)  to  protect  it. 

The  thimble  and  loop  is  formed  by  inserting  a  thimble 
in  the  loop,  winding  its  ends  A  (Fig.  75)  with  fine  copper 
wire,  skipping  a  couple 
of  spaces  B  and  C 
about  1/8  of  an  inch, 
cutting  the  end  D  of 
the  wire  at  a  slant  to 
taper  it  down  and 
thoroughly  soldering 


Fig.  75 


Fig.  76 


loop  and  thimble. 

The  wrapped  and 
soldered  loop  requires 
a  close  winding  of  copper  wire  around  the  part  to  be 
curved,  to  prevent  the  opening  of  the  coils  at  the  outside 
of  the  bend  A  (Fig.  76).  The  remainder  of  the  work  is 
done  as  in  the  thimble  and  loop. 

All  these  loops  are  fitted  with  shanks  before  being  closed. 

As  the  two  last  loops  are  soldered,  it  is  necessary  to 
know  the  soldering  process  to  be  able  to  properly  finish  the 
work. 

Soldering. — For  common  soft  soldering,  it  is  necessary  to 
have:  solder,  flux,  blow  torch  or  blow  furnace  and  soldering 
iron. 

The  solder  is  an  alloy  of  tin  and  lead,  usually  in  equal 
parts,  and,  therefore,  known  under  the  name  of  "Half  and 
half."  Sometimes,  more  tin  is  used  and  the  solder  is  then 
harder. 

Flux  is  a  substance  that  promotes  the  fusion  of  metals, 
prevents  their  oxidation  under  the  action  of  heat  and  cleans 
their  surface.  It  may  be  solid,  in  paste  form  or  liquid.  That 


106  AVIATION 

used  for  soft  soldering  is  generally  chloride  of  zinc,  which  is 
formed  by  dissolving  zinc  in  hydrochloric  acid. 

Hydrochloric,  or  muriatic  acid,  as  it  is  commonly  called, 
is  a  colorless,  corrosive  gas,  having  a  sharp  penetrating  taste 
and  suffocating  smell.  It  is  exceedingly  soluble  in  water 
and  when  it  comes  in  contact  with  the  air,  it  condenses  the 
moisture,  forming  dense,  white  clouds. 

What  is  commercially  known  as  hydrochloric  acid  is  a 
strong  aqueous  solution,  colored  yellow  by  impurities,  such 
as  iron  or  organic  substances.  Pure  hydrochloric  acid  is  a 
solution  of  pure  gas  in  distilled  water  and  is  colorless.  A 
concentrated  solution  of  hydrochloric  acid  gives  off  fumes 
when  exposed  to  the  air,  and  when  heated,  the  gas  evaporates. 

The  commercial  acid  is  obtained  in  the  soda  factories  by 
pouring  strong  sulphuric  acid  on  common  salt,  which  gives 
the  following  reaction: 

2  Na  Cl   +  H2  S  O4  =  Na2  S  O4     +     2  H  Cl 
Sodium         Sulphuric         Sodium         Hydrochloric 
chloride  acid  sulphate  acid 

The  hydrochloric  acid  thus  given  off  passes  through  special 
towers,  over  whose  walls  flows  a  constant  current  of  water, 
which  dissolves  the  gas  and  collects  it  at  the  bottom  of  the 
towers. 

To  prepare  the  flux,  zinc  is  treated  with  hydrochloric  acid, 
which  gives  the  following  reaction: 

Zn      +      2  H  Cl     =    Zn  C12     +     H2 
Zinc        Hydrochloric        Zinc        Hydrogen 
acid  chloride 

This  is  known  to  the  trade  as  "cutting  the  acid,"  which 
is  considered  "raw"  before  being  cut;  but,  as  we  see  from 
the  chemical  combination,  what  really  takes  place  is  the 
formation  of  chloride  of  zinc,  which,  if  used  as  flux,  prevents 
the  oxidation  of  metals  under  the  action  of  heat.  To  add 
to  this  property  of  the  flux  that  of  cleaning,  a  few  drops  of 


MAINTENANCE 


107 


raw  acid  are  poured  in  it.  As  the  solution  must  be  saturated, 
a  greater  quantity  of  zinc  is  used  than  that  necessary,  to 
make  sure  that  there  is  no  more  needed,  and  the  surplus  is 
removed  after  the  combination  ceases,  which  is  indicated  by 
the  stopping  of  the  bubbling  caused  by  the  reaction.  The 
acid  to  be  cut  must  be  poured  in  an  enameled  earthen  cup, 
as  there  is  development  of  heat  during  the  combination  and 
if  a  glass  vessel  were  used,  it  would  crack. 

A  blow  torch  is  a  lamp  that  burns  gasoline  mixed  with 
air  to  give  a  hot,  blue  flame.  Its  parts  are:  a  tank  A  (Fig.  77) 
which  contains  the  gasoline,  a 
hand  pump  B  to  force  and  com- 
press the  air  in  the  tank,  a  needle 
C,  a  needle  valve  D,  a  holed 
burner  E  which  mixes  the  gaso- 
line vapor  with  the  air  and  gives  a 
blue  flame,  a  drip  cup  F  in  which 
gasoline  is  burned  to  heat  the 
burner,  a  filler  plug  G  to  close  the 
tank,  and  a  tube  H  through  which 
the  gasoline  is  forced  up  to  the 
needle  valve  by  the  air  pressure. 


Fig.  77 


To  make  the  torch  ready  for  use,  it  is  turned  upside  down, 
the  filler  plug  at  the  bottom  unscrewed  and  the  tank  filled 
about  %  full  with  clean  gasoline.  To  prevent  a  leak,  it  is 
better  to  first  soap  the  threads  of  the  filler  plug  and  then 
screw  it  tightly,  using  a  wrench  or  an  iron  bar  inserted  in 
its  hole  to  make  sure  that  the  joint  is  air-tight,  but  care 
should  be  exercised  not  to  screw  the  plug  unnecessarily  hard, 
otherwise  the  bottom  of  the  "tank  will  be  distorted  and 
damaged,  being  very  thin. 

The  torch  is  turned  right  side  up  and  a  good,  heavy  pres- 
sure of  air  supplied  in  the  tank  by  means  of  the  pump.  A 
pressure  of  about  15  pounds  is  enough  for  a  strong,  blue 
flame,  but  the  higher  the  pressure,  the  better  the  flame.  If 
the  plunger  of  the  pump  is  of  the  kind  that  can  be  screwed 


108  AVIATION 

down,  it  must  be  screwed,  as  it  has  a  needle  point  on  its 
inner  end  to  make  a  positive  shut  off. 

The  tank  is  now  to  be  tried  for  leaks,  as  a  leaky  tank  not 
only  gives  a  poor  flame,  but  it  may  also  cause  a  fire  on  ac- 
count of  the  gasoline  oozing  out  through  the  leak  and  igniting. 
For  this  purpose,  the  torch  is  turned  upside  down;  this 
brings  the  gasoline  to  the  top  part  of  the  tank  and  in  case 
there  is  a  leak,  it  is  easily  found  and  stopped. 

If  there  is  no  leak  in  the  tank  or  fitting,  the  torch  is  turned 
right  side  up  and  the  drip  cup  filled  with  gasoline  by  putting 
one  hand  against  the  mouth  of  the  burner  and  opening  the 
needle  valve  with  the  other;  the  gasoline  strikes  the  hand, 
falls  in  the  burner  and  from  there  drops  in  the  drip  cup. 
When  this  is  full,  the  needle  valve  is  closed,  the  gasoline 
lighted  and  the  torch  protected  from  currents  of  air,  so  that 
the  burner  may  be  thoroughly  heated  by  the  flame.  Usually 
the  amount  of  gasoline  in  the  drip  cup  is  enough  to  heat  the 
burner  properly,  but  sometimes,  especially  in  cold  weather, 
it  is  not  sufficient,  and  in  this  case,  it  is  necessary  to  heat 
the  burner  by  means  of  a  flame  (either  from  another  torch 
or  a  gas  flame),  as  it  is  impossible  to  put  more  gasoline  in 
the  cup,  which,  being  warm,  causes  it  to  vaporize. 

If  the  tank  has  been  previously  tried  for  leaks  and  none 
found,  then,  to  fill  the  drip  cup,  only  a  few  strokes  of  the 
pump  will  suffice  to  supply  enough  pressure  to  force  the 
gasoline  out  slowly,  when  the  needle  valve  is  opened,  and 
fill  the  drip  cup  without  the  need  of  closing  the  mouth  of 
the  burner  by  the  hand.  Then  the  full  amount  of  air  pressure 
will  be  supplied  to  the  tank.  The  drip  cup  may,  of  course, 
be  filled  from  a  separate  source  than  the  tank. 

When  the  gasoline  is  nearly  burned  out  of  the  cup,  the 
needle  valve  is  slightly  opened  and  the  jet  of  gasoline  ignites. 
If  it  does  not  light  from  the  flame  of  the  cup,  it  must  be  lighted 
from  the  end  of  the  burner  and  the  flame  allowed  to  burn 
in  the  burner  tube  as  well  as  in  the  drip  cup.  When  the 
gasoline  is  all  burned  out  of  the  drip  cup,  the  valve  is  opened 


MAINTENANCE  109 

a  little  more  and  the  flame  allowed  to  burn  low  until  the 
burner  becomes  thoroughly  heated,  then  the  valve  is  opened 
enough  to  give  the  desired  flame. 

A  flame  about  4  inches  long  for  a  quart  and  3  inches  for 
a  pint  torch  generally  gives  the  best  results. 

When  through  using  the  torch,  the  valve  must  be  closed 
only  sufficiently  to  extinguish  the  flame  without  using  force, 
which  enlarges  the  needle  valve  and  ruins  the  burner.  After 
the  flame  is  out,  the  valve  must  be  opened  again  for  about 
one-quarter  of  a  turn  of  the  needle.  This  is  done  to  prevent 
damaging  the  needle  opening,  because  the  heat  causes  it  to 
expand,  and  when  it  cools  down  and  contracts,  if  there  is 
not  enough  room  allowed  for  the  contraction,  it  presses  so 
tightly  against  the  needle  that  it  is  hard  to  open  and  the 
consequent  friction  spoils  the  opening,  rendering  the  torch 
useless. 

The  principle  on  which  the  torch  works  is  this. 

The  pressure  of  air  in  the  tank  forces  the  gasoline  out 
through  the  tube  H  and  the  valve  Z),  and,  as  the  jet  rushes 
in  the  hot  burner  E,  it  is  vaporized  and  mixed  with  the  air 
sucked  in  through  the  holes  of  the  burner  by  the  current 
formed  by  the  jet  of  gasoline.  The  mixture  of  air  and  gasoline 
gives  a  hot,  blue  flame. 

From  this,  it  is  clear  why  the  tank  must  not  be  filled 
entirely  with  gasoline,  being  necessary  to  leave  room  for 
the  air,  so  that  a  high  pressure  can  be  brought  to  bear 
against  the  surface  of  the  gasoline,  because  the  greater  the 
pressure,  the  more  powerful  the  jet  through  the  burner,  the 
greater  the  suction  produced,  the  better  the  mixture  of  air 
and  gasoline,  and  the  better  the  flame.  To  make  sure  that 
some  air  will  be  in  the  tank  even  when  it  is  filled  full,  its 
bottom  is  made  funnel-shaped,  so  that,  besides  facilitating 
the  filling,  it  renders  impossible  the  expulsion  of  all  the  air 
from  it,  although  the  amount  left  in  is  much  less  than  that 
needed. 

If  the  torch  does  not  work,  the  tank  must  be  examined 


110  AVIATION 

to  see  that  the  filler  plug  is  screwed  in  tight,  that  there  is  the 
proper  pressure,  the  right  quantity  of  gasoline  and  no  leaks. 
If  the  burner  smokes  or  does  not  give  a  blast  when  the 
tank  is  tight  and  the  air  pressure  good,  it  indicates  that  the 
burner  is  dirty  or  clogged,  in  which  case,  it  must  be  taken 
out  and  cleaned  from  the  carbon  formed  around  it  as  well 
as  in  the  air  holes  of  the  burner.  In  reassembling,  the  joints 
must  be  soaped  to  prevent  leaks. 

If  the  washer  of  the  filler  plug  is  old  or  worn  out,  it  must 
bo  replaced  with  a  new  one  made  of  leather  or,  if  leather  is 
not  available,  with  a  soaped  cotton  string  wound  around  the 
plug  to  the  right,  so  that  when  the  plug  is  screwed  in  place, 
it  will  tighten  the  string. 

The  air  pump  must  be  oiled  often  to  keep  the  cup  leathers 
soft,  as  the  pump  heats  in  use  and  the  leather  dries,  causing 
the  pump  to  work  badly.  A  few  drops  of  oil  at  a  time  and 
often  will  keep  the  pump  in  good 
condition  and  increase  its  life. 

When  the  torch  is  not  in  use,  it 
must  not  be  stored  in  a  damp  place 
or  allowed  to  remain  in  contact  with 
acid  fumes,  as  it  will  oxidize. 

The  gasoline  used  for  torches  should 
Fig.  78  fa  ciean  and  kept  in  clean  cans  to 

avoid  stopping  the  burner,  and  it  should  be  of  good  quality 
to  give  the  best  results. 

A  furnace  (Fig.  78)  works  on  the  same  principle  as  a  torch, 
but  it  differs  in  size,  shape  and  material;  being  larger,  flatter 
and  made  of  iron  with  a  few  parts  of  brass,  while  the  torch 
is  mostly  brass. 

Although  the  make  of  a  furnace  varies  and  special  instruc- 
tions are  furnished  by  the  manufacturers,  generally  speaking, 
the  following  rules  apply  to  the  majority  of  them: 

If  the  burner  is  rigid,  it  will,  of  course,  be  always  in  the 
right  position,  but  if  it  is  mounted  on  a  swivel,  it  must  be 
placed  in  a  horizontal  position  to  fill  the  drip  cup,  and  the 


MAINTENANCE  111 

needle  opened  only  enough  for  the  gasoline  to  fall  in  the 
cup. 

The  swivel  action  may  cause  a  leak  at  the  shoulder  of  the 
needle,  in  which  case  the  stuffing  box  must  be  tightened. 

To  remove  the  burner,  it  is  necessary  to  remove  first  the 
top  chamber  by  turning  the  burner  so  that  it  stands  upright, 
unscrewing  the  thumbscrew  in  the  center  of  the  chamber 
and  lifting  it  off;  then  the  burner  and  swivel  are  unscrewed 
from  the  standpipe,  without  taking  apart  the  swivel  at  the 
union,  as  not  only  it  is  unnecessary,  but  it  may  cause  a 
leak,  if  it  is  not  properly  reassembled. 

To  clean  the  burner,  it  must  be  taken  off  together  with 
the  swivel,  the  spiral  core  removed  and  cleaned  thoroughly, 
forcing  out  all  dirt  from  the  hole  in  the  center  of  the 
core  by  means  of  a  fine  wire,  and  washing  the  burner  in 
gasoline.  If  the  wire  strainer  cloth  at  the  small  end  of  the 
swivel  is  dirty,  it  must  be  removed  and  replaced  with  a  new 
one,  made  of  the  proper  kind  of 
strainer  cloth,  rolled  up  tightly  and 
forced  into  the  hole,  to  keep  out  lint 
and  dirt  and  save  cleaning  the  burner 
oftener,  and  also  to  facilitate  the 
vaporization  of  the  gasoline. 

In  putting  the  burner  back,  care 
must  be  taken  to  set  it  in  the  proper 
position,  so  that  the  gasoline  can  fill 
the  drip  cup. 

If  there  is  a  leak  around  the  pump  collar,  caused  by  the 
washer  being  old,  it  must  be  renewed. 

The  so-called  soldering  iron  consists  in  reality  of  a  quad- 
rangular prism  of  copper  terminating  in  a  pyramid  A  (Fig. 
79),  a  wooden  handle  B  and  an  iron  bar  C  uniting  both. 

To  prepare  the  iron  for  use,  it  is  heated  to  a  temperature 
just  short  of  redness,  its  sloping  end  quickly  filed  bright, 
dipped  momentarily  into  the  flux  and  tinned  by  rubbing 
it  on  solder  laid  on  a  piece  of  tin  or  a  hard  wooden  block. 


112  AVIATION 

The  iron  may  be  filed  cold  or  warm,  but  it  is  better  to  file 
it  warm,  because,  in  this  state,  a  mere  shaking  of  the  iron 
causes  the  old  solder  to  drop  off,  and  a  few  quick  strokes  of 
the  file  are  enough  to  make  the  point  bright  without  loss 
of  heat.  If,  instead,  it  is  filed  when  cold,  it  takes  longer,  the 
work  is  harder,  the  solder  sticks  to  the  file  and  spoils  it, 
and  after  the  iron  is  heated,  it  is  necessary  to  file  it  again  to 
remove  the  oxidation  caused  by  the  heat.  In  reheating  the 
iron,  after  it  has  lost  its  proper  temperature  during  the 
soldering  process,  care  should  be  taken  not  to  heat  it  quite 
to  redness  or  the  solder  may  burn  off,  necessitating  a  repeti- 
tion of  the  tinning,  nor  to  overheat  it  to  such  a  temperature 
that  the  iron  burns,  and  must  be  filed  again,  which  means 
hard  work,  loss  of  metal  and  time. 

The  capital  requirement  for  true  soldering  is  that  between 
the  metal  to  be  soldered  and  the  solder  used  there  should  be 
a  certain  degree  of  alloying,  an  intimate  union  of  the  two 
thus  taking  place.  Beside  this,  it  is  necessary  that  the  metal 
be  bright,  clean  and  free  from  greasy  matter,  and  that  it 
be  coated  with  flux  to  prevent  its  oxidation  under  the  action 
of  heat. 

When  chloride  of  zinc  is  used  as  flux,  the  metal  soldered 
must  be  thoroughly  washed  to  remove  any  trace  of  acid, 
which  in  due  time  would  corrode  it.  For  this  reason,  the 
loops  that  require  soldering  are  dangerous,  because  the  acid 
can  not  be  all  removed;  but  as  long  as  they  are  still  in  use, 
it  is  well  to  know  how  to  solder  them. 

A  loop  is  first  coated  with  flux  and  soldered  on  both  sides 
with  more  solder  than  necessary;  then  it  is  laid  on  a  hot 
soldering  iron  and  pulled  slowly  along  it,  so  that  the  heat 
melts  the  solder  and  causes  it  to  filter  through;  and  finally 
it  is  thoroughly  washed  with  plain  or  soaped  water.  If  the 
work  is  properly  done,  when  the  loop  is  cut,  the  wire  looks 
as  if  it  were  solid  instead  of  stranded. 

The  openings  left  in  the  copper  wire  winding  are  for  the  pur- 
pose of  inspection,  that  is,  to  see  if  the  solder  filtered  through. 


MAINTENANCE 


113 


From  this  process,  it  is  clear  that  the  flux  remains  inside 
the  wire,  the  surrounding  solder  making  it  impossible  to 
wash  it  out,  and,  consequently,  it  will  exercise  its  corrosive 
action  without  being  detected  until  the  .wire  breaks. 

Fabric. — The  repairs  in  the  fabric  consist  in  sewing  plain 
tears  or  patching  holes.  In  either  case,  it  is  first  necessary 
to  remove  the  old  dope  by  rubbing  it  off  with  a  cloth  moist- 
ened with  acetone  or  by  applying  on  it  fresh  dope  and  allow- 


i 


c 

Fig.  80 

ing  it  to  cut  the  old  one,  when  it  is  removed  by  means  of  a 
piece  of  cloth. 

A  plain  tear  is  sewed  with  a  baseball  stitch  (Fig.  80a), 
that  is,  a  needle  filled  with  single  thread  is  passed  through 
the  tear,  stuck  from  underneath  in  one  side  A  of  the  torn 
fabric  and  pulled  out,  so  that  the  knot  remains  unseen  under 
the  fabric,  then  the  needle  is  inserted  again  in  the  cut,  stuck 
in  the  other  side  B  of  the  cut  and  pulled  out,  and  so  on. 
When  the  sewing  is  finished,  the  thread  is  cut  off  and  the 
end  tucked  under  the  tear,  which  is  doped,  covered  with  a 
patch  large  enough  to  hide  it  (Fig.  806)  and  the  patch  doped 


114 


AVIATION 


also.  After  this  is  dry,  another  patch  with  frayed  edges 
(Fig.  80c)  is  applied  on  it  and  coated  with  the  regular  number 
of  coats  of  dope  and  spar  varnish.  The  last  patch  is  frayed, 
because  the  frays  adhere  better  than  the  plain  edges. 

To  repair  a  hole  (Fig.  81a),  it  must  be  filled  in  with  a  piece 
of  fabric,  and  to  facilitate  the  work,  the  hole  is  cut  into  a 
regular  shape  with  square  corners,  which  are  then  slit  (Fig. 
816)  and  the  fabric  tucked  underneath  (Fig.  81c)  to  double 


Fig.  81 

it  up  and  make  it  stronger  at  the  edges,  which  are  to  be 
sewed.  A  piece  of  fabric  is  cut  of  such  a  size  that  when  its 
edges  are  tucked  under  on  all  sides,  it  is  about  1/16  of  an 
inch  smaller  than  the  hole  all  around  (Fig.  8ld)  to  make 
possible  the  stretching  of  the  fabric  in  sewing  it.  The  corners 
of  the  fabric  are  now  fixed  in  place  by  stitches  (Fig.  Sle)  to 
ascertain  that  the  patch  is  the  proper  size  and  to  render 
easier  its  sewing,  which  is  done  with  a  baseball  stitch  as 
for  a  plain  tear  (Fig.  8 1/)  and  the  work  finished  in  the  same 
way,  that  is,  by  doping  on  it  a  plain  and  a  frayed  patch 
(Fig.  81#)  and  giving  the  latter  the  required  number  of 
coats  of  dope  and  spar  varnish. 


MAINTENANCE  115 

Rubber. — A  puncture  in  the  inner  tube  of  a  tire  may  be 
repaired  in  an  emergency  case  by  means  of  a  prepared  patch, 
which  is  a  disk  of  rubber  covered  with  rubber  cement  pro- 
tected by  a  cloth.  To  make  the  patch  ready  for  use,  the 
cloth  is  removed,  gasoline  poured  on  the  cement  and  the 
patch  left  in  this  way  for  about  15  or  20  minutes.  The  tube 
around  the  puncture  is  washed  with  gasoline,  sandpapered, 
coated  with  rubber  cement  and  covered  with  the  patch, 
which  is  held  firmly  against  it  by  means  of  a  weight  until 
it  is  dry. 

If  the  puncture  is  so  small  that  it  can  not  easily  be  seen, 
the  tube  is  inflated  and  dipped  in  water:  the  bubbles  formed 
by  the  escaping  air  indicate  its  location.  The  tube  must,  of 
course,  be  dry  before  applying  the  patch,  which  must  be 
handled  by  the  edges  to  avoid  touching  the  cement  with 
fingers  soiled  with  oil  or  grease  and  spoiling  its  adhesive 
property. 

If  a  steel  brush,  instead  of  sandpaper,  is  used  to  rub  the 
tube,  care  must  be  taken  to  see  that  the  wires  are  all  even, 
as  sometimes  one  of  them  protrudes  more  than  the  others 
and  cuts  the  rubber  all  over. 

A  punctured  outer  casing  or  shoe  is  properly  repaired  by 
the  vulcanizing  process,  which  requires  the  skillful  use  of 
special  apparatus,  but  the  puncture  can  be  temporarily 
stopped  by  means  of  one  of  the  clamps  made  for  this  purpose 
and  inserted  in  the  casing  against  the  puncture,  to  prevent 
the  inner  tube  from  blowing  out  through  it.  As  a  substitute 
for  a  clamp,  a  piece  of  thick  rubber,  leather,  linoleum  or 
anything  stiff  enough  to  stop  the  puncture  temporarily  may 
be  used. 


CHAPTER  VI 
FLIGHT  HINTS 

Methods  of  Instruction. — This  chapter  is  not  meant  to 
teach  anybody  how  to  fly — flying  " by  correspondence"  is 
not  a  possibility — but  it  simply  aims  at  giving  a  brief  ex- 
planation of  how  the  theory  of  flight  is  applied  in  practice. 

Flying  can  be  learned  in  three  different  ways,  but  in  each 
of  them  a  previous  thorough  knowledge  of  the  elementary 
theory  of  flight,  aeroplane  construction  and  gas  engines  is 
essential.  With  this  premise,  let  us  examine  these  three 
different  methods. 

A  man  that  owns  an  aeroplane  and  has  at  his  disposal  a 
good  stretch  of  level,  clear  ground,  suitable  for  the  initial 
runs  up  and  down,  may  learn  slowly  and  gradually  the  use 
of  the  controls  before  he  attempts  to  leave  the  ground.  This, 
thoroughly  accomplished,  enables  him  to  make  low,  short 
jumps  which  are  gradually  increased,  first  to  longer  and 
longer  straight,  low  flights  and  then  to  higher  and  higher 
altitudes,  until  he  has  learned  perfectly  the  use  of  all  the 
controls,  so  that  their  manipulation  becomes  almost  in- 
stinctive. Then  he  can  attempt  cross  country  and  high 
altitude  flights  and  the  performance  of  all  the  tricks  or  stunts 
that  go  to  make  the  expert  aviator.  But  while  this  can  be 
and  has  actually  been  done  by  the  pioneers  of  aviation,  or 
no  man  would  be  flying  to-day,  as  no  man  was  born  a  bird, 
on  the  other  hand,  it  implies  the  courting  of  all  the  risks  and 
dangers,  often  fatal,  encountered  by  all  those  who  made 
human  flight  an  actual  fact.  Why,  then,  take  such  perilous 
chances  when  we  have  to-day  plentj7"  of  expert  instructors, 
who  can  teach  us  with  the  minimum  danger?  And  it  is  the 
employment  of  an  instructor,  coupled  with  the  type  of 

116 


FLIGHT  HINTS  117 

machine  used,  which  gives  us  the  other  two  systems  of 
learning  how  to  fly. 

If  a  one-seat  machine  is  used,  as  was  the  case  before  the 
two-seat  machine  was  built,  the  instructor  explains  to  the 
pupil  the  manipulation  of  the  controls  and  mechanical  de- 
vices of  the  motor  and  makes  him  execute  the  different 
motions,  first  with  the  machine  stationary  and  then  running 
up  and  down  the  field,  until  he  has  mastered  the  manipula- 
tion of  the  controls  and  motor  mechanisms.  This  instruc- 
tion is  followed  by  the  flying  lessons,  which  are  first  explained 
and  practically  demonstrated  by  the  instructor  and  then 
executed  by  the  pupil.  From  the  .foregoing,  it  is  easy  to 
see  that  the  pupil  is  always  alone  in  handling  the  machine, 
and  the  corrections  can  be  made  only  after  the  run  on  the 
ground  or  the  flight  is  finished.  This,  undoubtedly,  intro- 
duces an  element  of  danger,  which,  to  be  minimized,  implies 
a  slow,  gradual  course  of  instruction  with  the  consequent 
loss  of  time.  While  the  sponsors  of  such  method  admit 
this  fault,  they  claim  in  its  favor  the  thorough  confidence 
gained  by  the  pupil,  who,  being  left  upon  his  own  resources 
right  from  the  beginning,  will  never  have  any .  trouble  in 
solving  his  own  problems  at  any  time,  no  matter  how  hard 
they  may  be.  While  there  is  truth  in  this,  the  loss  of  time 
involved  and  the  ever  present  source  of  irreparable  injury 
or  death  certainly  do  not  militate  in  its  favor. 

The  quickest,  best  and,  above  all,  safest  method  is  the 
dual  control  system;  that  is,  the  use  of  a  two-seat  machine 
with  duplicate  controls,  which  can  be  used  by  two  persons 
contemporaneously.  This  means  that  the  pupil  is  never 
alone,  but  has  always  ready,  to  keep  him  out  of  trouble, 
the  helping  hand  of  his  instructor,  thus  eliminating  alto- 
gether the  possibility  of  injury,  due  to  faulty  handling  of 
the  machine.  The  instructor  first  executes  a  given  maneuver 
and  the  pupil,  having  the  other  set  of  controls  at  his  dis- 
posal, feels  all  the  motions  made  by  the  instructor  and  learns 
practically  how  to  make  them  himself;  then  the  instructor 


118  AVIATION 

allows  him  to  repeat  the  same  performance  and  in  case  he 
errs,  he  is  instantly  corrected,  thus  learning  the  proper  way 
of  handling  the  controls.  Formerly,  oral  tuition  was  im- 
possible during  flight,  due  to  the  roar  of  the  motor,  and  it 
was  given  before  and  after  taking  the  air,  but  with  the  in- 
vention of  the  aerotelephone,  even  this  fault  has  disappeared, 
and  instructor  and  pupil  are  now  in  constant  verbal  com- 
munication. It  is  clear  that  this  is  by  far  the  best  method 
of  teaching  how  to  fly,  and  one  which  can  hardly  be  improved 
as  long  as  a  noisy  motor  is  used  to  furnish  the  motive  power. 
Still,  there  are  those  who  object  to  this  system  of  tuition  on 
the  ground  that  it  causes  lack  of  confidence  in  the  pupil, 
who,  when  left  alone,  doubts  his  own  ability  to  handle  the 
machine  and  finds  himself  in  trouble,  which  rnay  bring  serious 
consequences.  While,  admittedly,  it  is  human  to  become 
nervous  in  a  case  like  this,  on  the  other  hand,  the  lack  of 
confidence  in  the  pupil  is  due  more  to  the  fault  of  the  in- 
structor, and  to  a  certain  extent  to  the  pupil's,  than  to  tlie 
system.  If  the  instructor  is  really  worthy  of  the  name,  he 
will  not  consider  himself  all  the  time  the  master  of  the  situa- 
tion, but  will,  after  he  has  properly  taught  his  pupil,  allow 
him  to  handle  the  machine  all  by  himself,  being  ready  to 
come  to  the  rescue  only  in  case  of  an  emergency;  in  other 
words,  the  instructor  will  take  the  part  of  a  mere  passenger 
and  allow  the  pupil  to  be  the  pilot.  This  will  increase,  rather 
than  decrease,  the  confidence  in  the  pupil,  because  he  will 
attempt  all  the  different  maneuvers  with  the  assurance  that 
if  he  goes  wrong,  no  harm  will  befall  him,  and  thus  he  will 
acquire  a  thorough  knowledge  of  practical  flying,  which  he 
will  be  ready  to  use  at  any  time  afterwards,  be  he  in  com- 
pany or  alone.  If  he  were,  instead,  his  self-instructor  in  a 
one-seat  machine,  would  he  be  better  off,  would  he  dare 
execute  any  of  the  more  difficult  feats  of  daring,  which  were 
never  taught  him  in  a  practical  way?  Evidently,  in  such  a 
case,  he  has  to  take  a  chance,  but  so  did  others  before  him 
and  often  were  maimed  or  killed.  If  he  is  his  self -instructor, 


FLIGHT  HINTS  119 

he  must  proceed  slowly,  gradually,  carefully;  and  this  is 
exactly  his  part  with  the  dual  control  system  when  he  is 
left  alone  to  fly,  if  his  instructor  did  not  teach  him  properly. 
Having  practically  mastered  every  phase  of  the  evolutions 
through  the  guidance  of  the  instructor,  when  the  pupil  takes 
the  air  alone,  he  has  to  go  over  one  by  one  all  the  different 
manipulations,  from  the  easiest  to  the  most  difficult,  using 
good  common  sense  in  everything  he  does,  if  he  wants  to 
become  a  skillful  aviator  with  the  minimum  of  risk. 

No  matter  what  method  used,  the  course  of  instruction  is 
the  same;  that  is,  after  having  mastered  an  elementary, 
but  thorough  knowledge  of  the  theory  of  flight,  aeroplane 
construction  and  gas  engines,  the  pupil  is  taught  successively: 
taxying  or  grass  cutting,  elementary  flying,  stunts. 

Taxying. — Taxying  is  the  running  of  an  aeroplane  on  the 
ground,  and  it  has  the  object  of  familiarizing  the  pupil  with 
the  manipulation  of  the  controls  and  motor  devices.  The 
machine  used  for  taxying  is  either  a  heavy  one  unable  to 
leave  the  ground  or  a  regular  machine  with  the  motor  so 
adjusted  that  the  power  developed  is  insufficient  for  flight. 

In  the  case  of  the  stick  control,  the  manipulations,  as  we 
already  know,  are  the  following: 

The  stick  pushed  down  causes  the  nose  of  the  machine 
to  go  down;  the  stick  pulled  up  causes  the  nose  to  go  up. 

The  stick  thrown  to  the  right  brings  down  the  right  side 
of  the  machine;  the  stick  thrown  to  the  left  brings  down  the 
left  side  of  the  machine. 

The  foot  rudder  bar  pushed  to  the  right  makes  the  ma- 
chine turn  to  the  right;  the  foot  rudder  bar- pushed  to  the 
left  makes  the  machine  turn  to  the  left. 

With  the  wheel  control,  the  only  difference  consists  in 
turning  the  wheel  to  the  right  or  left  to  accomplish  the  same 
result  as  when  the  stick  is  thrown  to  the  right  or  left. 

One  very  important  point  to  impress  on  the  mind  of  the 
pupil  right  from  the  beginning  is  to  hold  the  control  lever 
naturally,  without  an  undue  amount  of  force,  and  to  handle 


120  AVIATION 

it  lightly  and  slowly  to  acquire  the  proper  touch,  and  control 
the  machine  in  the  right  way.  The  motions  of  the  controls 
during  flight  are  almost  imperceptible,  especially  with  a 
very  sensitive  machine. 

The  controls  are  first  operated  by  the  pupil  while  the 
machine  is  stationary  and  then  the  manipulations  are  re- 
peated with  the  machine  in  motion  on  the  ground  under  its 
own  power.  Usually,  the  machine  runs  in  a  straight  line 
from  one  end  of  the  aviation  field  to  the  other,  then  the 
motor  is  throttled  down  or  stopped  and  the  machine  turned 
around  by  hand  to  repeat  the  run;  but  sometimes  the  turn 
is  made  under  power  by  manipulating  the  controls  accord- 
ingly. 

There  is  quite  a  difference  between  handling  a  machine 
on  the  ground  and  in  the  air:  on  the  ground,  there  is  to  take 
into  consideration  the  friction  of  the  wheels  and  skid  against 
the  surface  of  the  earth,  which,  especially  in  a  badly  made 
turn  when  the  machine  is  under  power,  may  cause  the 
stripping  of  the  tires,  the  buckling  up  of  the  wheels  or  the 
smashing  of  the  undercarriage.  There  is  also  the  possibility 
of  breaking  the  wings  by  too  steep  a  bank,  which  causes  the 
wing  tips  to  come  in  contact  with  the  ground. 

When  the  pupil  becomes  proficient  in  handling  the  con- 
trols, he  is  taken  up  in  the  air  in  a  dual  control  machine  and 
instructed  in  the  art  of  flying. 

Elementary  Flying. — Here  is  the  proper  time  to  point  out 
something  really  bad,  which  is  possible  only  with  the  dual 
control  method  of  instruction  and  which  once  more  goes  to 
show  that  it  is  not  the  system  that  is  wrong,  but  the  instruc- 
tor— sometimes. 

There  are  so-called  expert  instructors — and  they  may 
really  be  experts  in  aviation,  but  not  by  any  means  in  psy- 
chology— who  take  an  immense  delight  in  frightening  to 
the  highest  degree  the  poor  pupil  they  take  up  for  the  first 
time.  That  this  is  a  pernicious  habit,  which  ought  to  be 
stopped  or  punished,  if  possible,  is  not  necessary  to  empha- 


FLIGHT  HINTS  121 

size,  but  it  is  well  to  say  that  such  a  mischievous  trick  has 
sometimes  had  terrible  consequences,  which  by  themselves 
ought  to  be  sufficient  to  warn  the  would-be  silly  teaser.  In 
one  particular  instance,  an  instructor,  who  was  taking  up  a 
pupil  for  his  first  flight,  aimed  the  machine  straight  for  a 
hangar,  expecting  to  jump  over  it  within  the  least  distance — 
an  easy  thing  to  do  for  an  expert  aviator — but  the  pupil, 
thinking  the  instructor  had  gone  crazy,  unexpectedly  took  a 
hand  in  the  matter  and,  frightened  as  he  was,  operated  the 
controls  with  such  a  jerk  that  the  aviator  was  unable  to 
execute  the  necessary  maneuver  instantly  and  the  machine 
went  to  smash  itself  against  the  hangar.  This  was  the  re- 
sult of  a  mean  trick  on  the  part  of  a  man,  whose  duty  was 
just  the  opposite.  But  flying  is  safe  if  properly  done,  and 
a  great  deal  in  the  improvement  of  the  aeroplane  is  due  to 
the  war,  which,  horrible  as  it  has  been  in  all  other  respects, 
has  given  a  great  impetus  to  aviation,  on  account  of  the 
millions  spent  in  perfecting  the  aeroplane  as  an  engine  of  war. 

For  instruction  purposes,  when  a  single-seat  machine  is 
used,  the  best  suited  is  the  inherently  stable;  but  with  the 
dual  control,  it  is  better  to  use  a  machine  with  rather  high 
power  and  sensitive  controls. 

In  teaching  his  pupil,  the  instructor  first  executes  himself 
the  simplest  manipulations  and  then  tells  the  pupil  to  repeat 
them,  correcting  and  advising  him  constantly  with  his  viva 
voce  instructions. 

An  instructor  may,  for  instance,  proceed  by  having  the 
pupil  perform  the  different  maneuvers  in  the  following  order: 

Straight  flight.  This  will  teach  the  pupil  to  hold  the 
controls  in  the  neutral  position  and  experience  the  impossi- 
bility of  keeping  the  machine  level  and  on  a  straight  course, 
due  to  the  constantly  changing  currents  of  air,  without  the 
continuous  manipulation  of  the  controls. 

Slight  deviations  from  the  straight  course  by  pushing  the 
rudder  first  to  the  left,  which  is  easier  of  accomplishment,  and 
then  to  the  right,  so  that  he  may  get  used  to  turn  both  ways. 


122  AVIATION 

Slight  climbs  and  descents  to  get  used  to  the  manipula- 
tion of  the  elevators. 

Slight  sideways  motions  to  learn  how  to  operate  the  ail- 
erons. 

Steeper  climbs  and  descents. 

Sharp  left  and  right  turns  separately,  involving  the  con- 
temporaneous use  of  rudder  and  ailerons,  as  the  machine 
must  be  banked  in  a  sharp  turn  to  prevent  side  slipping  or 
skidding. 

Left  and  right,  right  and  left  turns  combined  so  as  to 
describe  a  figure  8. 

Slight  climbing  turns  to  combine  the  use  of  all  the  controls. 

Gliding  with  motor  throttled  down  and  with  the  motor 
stopped. 

Spiraling,  or  gliding  and  turning  in  the  meantime  in 
smaller  and  smaller  circles  with  the  power  shut  off. 

Ascending  from  the  ground. 

Landing  against  the  wind  and  across  the  wind. 

Notice  that  landing  is  at  the  bottom  of  the  list,  being  the 
most  difficult  thing  to  learn,  while  ascending  immediately 
precedes  it,  as  it  is  easier  than  landing,  but  harder  than  any 
aerial  maneuver. 

For  each  evolution  in  the  air,  the  instructor  teaches  the 
pupil  how  to  recover  from  it  and  bring  the  machine  back 
to  the  normal  position. 

When  the  pupil  can  handle  the  machine  perfectly  well, 
without  the  presence  of  the  instructor, — solo  flying — then 
he  will  be  taught  advanced  flying,  which  will  classify  him  as 
an  expert  aviator. 

VStunts. — The  capital  requirements  of  a  stunting  machine 
are  strength  and  sensitiveness  of  controls.  Of  course,  some 
of  the  stunts  can  be  made  with  relatively  slow  and  not  very 
strong  machines,  but,  on  all  occasions,  it  is  better  to  remem- 
ber the  wise  golden  rule:  Safety  first! 

Before  proceeding  to  explain  some  of  the  most  common 
stunts,  it  is  well  to  go  over  a  few  things  already  treated,  to 


FLIGHT   HINTS 


123 


add  and  explain  a  couple  of  new  terms  and  different  func- 
tions of  the  controls. 

The  heavier  and  slower  a  machine,  the  less  sensitive  the 
controls. 

Even  in  a  speedy  machine,  the  controls  become  less  effi- 
cient if  the  speed  slackens;  the  efficiency  of  the  controls  is, 
therefore,  directly  proportional  to  the  speed. 

As  all  water  machines  are  heavier  and  slower  than  land 
machines,  it  follows 
that  the  manipulation 
of  the  controls  differs, 
being  more  pronounced 
in  the  former  than  in 
the  latter. 

In  order  to  execute 
stunts  safely,  especi- 
ally for  the  beginner, 
it  is  necessary  to  at- 
tain a  good  altitude, 
because  whenever  the 
machine  is  not  in  its 
normal,  level,  straight  flight,  there  is  loss  of  lift  with  a  con- 
sequent sagging  effect. 

It  is  necessary  to  remember  how  the  absence  of  the  pro- 
peller torque,  when  the  motor  is  stopped,  affects  a  single- 
propeller  machine,  whether  it  has  or  not  wash  in  or  wash  out. 

In  case  of  a  very  steep  or  vertical  bank,  the  functions  of 
rudder  and  elevators  are  completely  reversed:  the  rudder 
A  (Fig.  82)  being  then  in  a  horizontal  position  will  be  used 
to  bring  the  nose  of  the  machine  up  or  down;  the  elevators  B 
being  vertical  serve  to  make  the  machine  turn  around. 

In  connection  with  steep  banks,  the  terms  used  for  the 
rudder  are:  top  rudder  and  bottom  rudder,  irrespective  of 
the  fact  that  in  either  case  it  may  be  right  or  left  rudder. 
For  instance,  if  the  machine  is  banked  so  that  the  right  wing 
A  (Fig.  83a)  is  down,  then  bottom  rudder  B,  in  this  case, 


Fig.  82 


124 


AVIATION 


would  be  equivalent  to  right  rudder;  but  if  the  case  is  re- 
versed, that  is,  if  the  machine  is  banked  with  the  left  wing 


:c 


Fig.  83 

C  (Fig.  83)  down,  then  bottom  rudder  D  would  mean  left 
rudder. 

If  we  first  consider  a  machine  in  its  normal  flying  position 


A 


Fig.  84 

(Fig.  84a),  but  with  the  wings  banked,  and  then  we  consider 
it  upside  down  (Fig.  84)  and  with  the  same  bank,  what 


FLIGHT  HINTS 


125 


in  the  first  case  is  considered  as  the  lower  wing  A  and  the 
higher  wing  B,  in  the  second  becomes  higher  A'  and  lower  Br. 
The  bank  is  corrected  in  both  cases  with  the  same  manipula- 
tion of  the  controls,  the  difference  being  that  in  the  first  case 
the  wing  would  be  depressed,  while  in  the  second  it  would 
be  raised. 

The    above    facts    must    be    taken    into    consideration 


Fig.  85 

whenever  handling  a  machine  either  in  regular  flight  or 
in  stunts. 

Now,  let  us  take  up  some  of  the  principal  stunts. 

Side  slip  (Fig.  85)  is  the  sideways  fall  of  an  aeroplane  due 
to  over  banking. 

To  side  slip:  bank  steeply. 

To  recover  from  a  side  slip:  move  the  control  lever  forward, 
throw  lever  towards  the  higher  side  of  the  machine  until  it  is 


126 


AVIATION 


level,  neutralize  the  lever,  pull  the  lever  back  gradually  to 
flatten  the  course  of  the  machine,  neutralize  the  lever. 


Fig.  86 

Spin  (Fig.  86)  is  a  sideways  whirling  fall  of  an  aeroplane 
due  to  underbanking  in  a  turning  motion.     This  gives  us 


FLIGHT  HINTS 


127 


the  clue  how  to  reproduce  it  voluntarily,  but  there  is  this 
important  point  to  remember:  when  a  spin  is  involuntary, 


Fig.  87 

the  motor  may  or  may  not  be  running,  and  in  case  it  is,  it  is 
wise  to  throttle  it  down  to  avoid  undue  speed  and  consequent 


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FLIGHT  HINTS  129 

stresses,  while  to  reproduce  a  spin  voluntarily,  the  first 
thing  to  do  is  to  throttle  down  the  motor. 

To  make  a  spin:  throttle  down  the  motor,  put  the  rudder 
on  sharply,  pull  back  the  control  lever. 

To  recover  from  a  voluntary  or  involuntary  spin:  neutral- 
ize the  rudder,  push  down  the  control  lever,  pull  back  the 
lever  gradually  to  flatten  the  course  of  the  machine,  neutral- 
ize the  lever. 

Tail  slide  (Fig.  87)  is  the  falling  of  an  aeroplane  tail  fore- 
most, caused  by  a  climb  up  to  the  stalling  angle. 

To  produce  a  tail  slide:  pull  the  control  lever  back  and 
climb  steeply  until  the  machine  stalls,  throttle  down  the 
motor.  The  machine  falls  tail  foremost  at  first,  but  then 
the  nose  drops  gradually  and  a  dive  begins. 

To  recover  from  a  tail  slide :  pull  back  the  lever  gradually 
when  the  machine  begins  to  dive  in  order  to  flatten  its  course, 
throttle  on  the  motor. 

Loop  (Fig.  88)  is  a  circle  described  by  an  aeroplane  through 
the  proper  manipulation  of  the  elevators. 

The  elevators  must  be  operated  properly  to  avoid  a  stall, 
and  they  must  be  assisted  by  the  rudder  and  ailerons  to  keep 
the  machine  on  a  straight,  level  course  to  prevent  a  side 
slip,  and  by  the  timely  throttling  down  of  the  motor  to 
avoid  a  tail  slide  or  too  wide  a  loop. 

To  loop:  climb  at  a  good  altitude,  dive,  pull  back  the 
control  lever  gradually  until  the  machine  is  in  a  vertical 
position,  pull  the  lever  all  the  way  back,  throttle  down  the 
motor  at  the  top  of  the  loop,  push  down  the  lever  just  short 
of  the  neutral  position  as  the  machine  begins  to  fall  nose 
first,  pull  the  lever  gradually  until  the  machine  flattens  out, 
throttle  on  the  motor. 

If  a  second  loop  is  to  be  made,  then  the  course  of  the  ma- 
chine is  not  flattened  out  at  the  end  of  the  first  loop,  but  it  is 
allowed  to  dive  until  it  acquires  the  necessary  speed  to  repeat 
the  looping  evolution. 


•s- 


130 


FLIGHT  HINTS  131 

Roll  (Fig.  89)  is  the  turning  motion  of  an  aeroplane  around 
its  longitudinal  axis. 

If  the  machine  is  to  turn  around  its  longitudinal  axis, 
it  is  clear  that  this  operation  must  be  brought  about  by 
means  of  the  ailerons.  If  the  machine  is  banked  and  kept 
banked  with  the  motor  running,  the  continued  operation  of 
the  ailerons  will  cause  it  to  turn  over  and  over,  thus  producing 
a  screw-like  action,  which  assists  the  forward  motion,  but  as 
there  is  loss  of  lift,  which  tends  to  make  the  machine  sag, 
it  is  necessary  to  aid  this  evolution  by  the  occasional  use  of 
the  rudder  and  elevators. 

To  make  a  complete  left  roll,  the  operation  may  be  sum- 
marized thus:  push  down  the  control  lever  and  then  pull 
it  up  so  that  the  machine  first  dives  and  then  climbs,  bank 
sharply  until  vertical,  put  on  bottom  rudder  to  assist  the 
rolling  motion,  neutralize  the  rudder,  pull  back  the  lever  to 
lower  the  nose  a  little  when  the  machine  is  upside  down,  put 
on  top  rudder  when  the  machine  is  again  vertical,  neutralize 
the  rudder  when  the  machine  returns  to  its  normal  position. 

This  completes  the  roll.  If  more  consecutive  rolls  are 
desired,  the  operation  is  repeated  over  and  over. 

The  process  to  be  followed  to  obtain  a  complete  right  roll 
differs  only  in  the  operation  of  the  rudder,  which,  instead  of 
being  first  bottom  and  then  top  rudder  as  in  the  present 
case,  will  be  reversed:  top  and  bottom  rudder. 


APPENDIX 
AERODYNAMICAL  FORMULA  AND  CALCULATIONS* 

My  study  of  aeronautical  science,  or  rather  my  battle  with  the 
text-books  on  aerodynamics,  has  been  the  longest,  hardest  mental 
struggle  of  my  life. 

Contrary  to  the  rule  for  the  mastery  of  knowledge,  the  more  I 
studied,  the  less  I  knew;  but,  luckily,  the  less  I  knew,  the  more  grew 
my  desire  to  know.  And  I  studied  all  the  aeronautical  books  I  could 
get  hold  of  and  written  in  the  languages  I  understand,  but  the 
result  was  simply  the  twentieth-century  reestablishment  of  the 
ancient  kingdom  of  Babylonia  right  between  my  brains,  and  an 
infernal  dance  of  angles,  sines,  cosines,  tangents  and  coefficients, 
which  brought  about  such  a  tremendous  pressure  against  the 
center  of  gravity  of  my  brain  as  to  threaten  to  unbalance  it  and 
to  render  myself  fit  for  a  straight  flight  right  into  an  insane  asylum. 
And  this  would  certainly  have  been  my  fate,  if  a  great  scientist 
did  not,  unknowingly,  come  to  my  help. 

This  great  scientist  is  the  illustrious  brother  of  our  illustrious 
President,  Sir  Hiram  Maxim. 

In  his  valuable  book,  "Artificial  and  Natural  Flight,"  I  found 
the  solution  of  the  hard  problem;  not  only  for  its  sound  teachings, 
but,  also,  for  its  emphatic  approval  of  another  book,  which  I  had 
already  studied  and  which,  as  all  others,  I  was  in  doubt  to  follow, 
until  so  high  an  authority  recommended  it  as  "the  most  elaborate 
and  by  far  the  most  reliable." 

One  thing  that  attracted  my  special  attention  in  studying  Sir 
Maxim's  book  was  the  fact  that,  in  my  trouble  in  regard  to  the  study 
of  aerodynamics,  I  was  in  good  company,  as  he  himself  had 
the  same  experience  when  he  first  started  to  learn  this  famous 
science. 

And  not  to  alter  the  peculiar  Maximian  style,  I  will  quote  his 
own  words. 

*  Lecture  delivered  before  the  Aeronautical  Society  of  New  York 
in  February,  1911. 

133 


134  APPENDIX 

"During  the  last  few  years,  a  considerable  number  of  text-books 
have  been  published.  These  have  for  the  most  part  been  prepared 
by  professional  mathematicians,  who  have  led  themselves  to  be- 
lieve that  all  problems  connected  with  mundane  life  are  susceptible 
of  solution  by  the  use  of  mathematical  formulae,  providing,  of  course, 
that  the  number  of  characters  employed  are  numerous  enough. 
When  the  Arabic  alphabet  used  in  the  English  language  is  not 
sufficient,  they  exhaust  the  Greek  also,  and  it  even  appears  that 
both  of  these  have  to  be  supplemented  sometimes  by  the  use  of 
Chinese  characters.  As  this  latter  supply  is  unlimited,  it  is  evi- 
dently a  move  in  the  right  direction.  Quite  true,  many  of  the 
factors  in  the  problems  with  which  they  have  to  deal  are  completely 
unknown  and  unknowable;  still  they  do  not  hesitate  to  work  out  a 
complete  solution  without  the  aid  of  any  experimental  data  at  all. 
If  the  result  of  their  calculations  should  not  agree  with  facts,  'bad 
luck  to  the  facts.'" 

In  another  part  of  his  book  he  says  further, 

"Some  of  the  mathematicians  have  demonstrated  by  formulae, 
unsupported  by  facts,  that  there  is  a  considerable  amount  of  skin 
friction  to  be  considered,  but  as  no  two  agree  on  this  or  any  other 
subject,  some  not  agreeing  to-day  with  what  they  wrote  a  year  ago, 
I  think  we  might  put  down  all  of  their  results,  add  them  together, 
and  then  divide  by  the  number  of  mathematicians,  and  thus  find 
the  average  coefficient  of  error." 

But  let  us  consider  this  controversy  as  a  thing  of  the  past,  and 
and  let  us  follow  Sir  Maxim's  teachings  and  recommendations, 
which  favor  the  use  of  Ritter  von  Loessel's  formulae. 

The  most  important  law  governing  aerial  flight  is  based  on  the 
resistance  of  the  air  against  a  plane  moving  through  it;  and  while 
all  scientitsts  agree  that  this  resistance  is  proportional  to  the  sur- 
face of  the  plane  and  to  the  square  of  its  velocity,  no  two  agree  on 
the  value  of  the  coefficient  of  resistance,  which  has  been  baptized  K. 

This  famous  or  infamous  coefficient  K,  as  a  French  writer  calls 
it,  or  this  ghost,  as  a  member  of  our  Society  most  appropriately 
named  it,  has  been  made  to  assume  values  ranging  from  55  to  132 
grammes  to  the  square  meter,  which,  as  you  see,  shows  very  little 
difference.  Anyhow,  according  to  the  latest  edition  and  from 
experiments  made  on  the  Eiffel  Tower,  the  value  of  the  coefficient 
K  is  made  equal  to  80  grammes  per  square  meter.  So,  barring 


APPENDIX  135 

further  editions  and  extras,  we  can  take  it  for  granted  that  we  know 
the  value  of  the  coefficient  of  air  resistance. 

Now,  then,  if  we  call  R  the  resistance  of  calm  air  against  a  plane 
moving  perpendicularly  through  it,  S  the  surface  of  the  plane  ex- 
pressed in  square  meters,  and  V  its  velocity  in  meters  per  second, 
we  obtain  the  formula: 


I  said  calm  air,  because  if  the  air  has  a  motion  of  its  own,  v,  then 
the  formula  will  be  : 


And  it  will  be  +v  if  the  air  moves  in  the  sense  of  the  motion  of  the 
plane,  —  v  if  in  opposite  sense,  and  Ov  if  in  calm  air. 

We  will  consider  only  this  last  case. 

This,  therefore,  gives  us  the  resistance  of  the  air  against  a  plane 
moving  perpendicularly  through  it,  but  in  this  way  we  could  not 
obtain  sustentation,  as  the  plane  simply  meets  the  air  squarely  in 
front  of  it  and  uses  up  all  its  energy  in  forcing  it  back,  without  pro- 
ducing useful  work.  What  we  need,  instead,  is  that  the  plane  be 
placed  in  such  a  position  that  the  air  give  up  some  of  its  energy 
to  the  plane.  Evidently,  to  produce  this  effect,  we  must  place 
the  plane  in  an  inclined  position,  so  that  the  air,  meeting  it  at  the 
forward  edge,  instead  of  being  forced  back,  is  compelled  to  flow 
awa}^  at  the  rear,  producing  useful  work. 

And  here  we  are  again  up  against  another  controversy,  which 
rivals  in  importance  that  of  the  coefficient  K]  that  is,  the  determina- 
tion of  the  resistance  of  the  air  against  an  inclined  plane,  according 
to  its  angle  of  incidence. 

Without  losing  time  in  discussing  Newton's  formula,  and  the 
opinions  given  in  favor  and  against  it  by  ae'ro-mathematicians  of 
the  present  day,  we  will  use  von  Loessel's  formula,  which  gives  the 
resistance  of  the  air  against  an  inclined  plane,  moving  through  it, 
as  being  proportional  to  the  sine  of  the  angle  of  incidence.  We  will 
have,  then,  the  formula: 

Ra  =  K  S  F2sino 

in  which  a  is  the  angle  of  incidence. 

Let  us  consider,  now,  the  plane  A  B,  moving  through  the  air  at 
an  angle  ABC. 


136  APPENDIX 

The  air  will  exervise  against  the  plane  a  certain  resistance  R, 
which  we  will  consider  as  resolved  in  two  forces:  R'  and  R":  the 


90 

first,  R',  tending  to  lift  the  plane,  and  the  second,  R",  resisting  the 
forward  motion. 

The  vertical  component  R'  is  the  lift  of  the  surface,  and  the  hori- 
zontal component  R"  is  the  drift. 

The  center  of  pressure  of  a  plane  surface  is  near  the  front  at  0° 
of  incidence,  and  it  travels  slowly  backward  as  the  angle  increases, 
until  it  reaches  the  middle  of  the  surface  at  90°. 

The  formula  which  will  enable  us  to  find  the  center  of  pressure 
of  a  plane  surface  moving  through  the  air  at  different  angles  of 
incidence  is: 

x  =(0.2 +0.3  sin  a)  I 

in  which  I  is  the  length  of  the  inclined  side. 
If  we  make  1=  1,  the  formula  will  be: 

x  =  0.2+0.3  sin  a 
If  sin  a  =  0, 

z  =  0.2+0.3XO  =  0.2, 
and  if  sin  a=l, 

x  =  0.2+0.3X1  =  0.5. 

At  0°,  then,  the  sine  being  0,  we  find  that  the  center  of  pressure 
is  at  2/10  from  the  front  edge;  and  at  90°  the  sine  being  1,  the 
center  of  pressure  is  at  5/10  from  the  front  edge;  that  is,  at  the 
center  of  figure,  and,  therefore,  the  center  of  figure  and  the  center 
of  pressure  coincide. 


FLIGHT  HINTS  137 

Calling  L  the  lift  and  D  the  drift,  we  may  resolve  the  formula: 

Ra  =  KSV*swa 
into  the  two  components: 

L  —  K  S  F2  sin  a  cos  a 
D  =  K  S  Vz  sin2  a 

Sir  Maxim  uses  different  and  practical  methods  to  find  out 
the  lift  and  drift,  but  his  considerations  agree  perfectly  with  von 
LoesseFs  formulae. 

Sir  Maxim  says  that  "the  lifting  effect  will  be  just  as  much 
greater  than  the  drift,  as  the  width  of  the  plane  is  greater  than  the 
elevation  of  the  front  edge  above  the  horizontal."  But  he  admits 
that  "as  the  front  edge  A  of  the  plane  A  B  is  raised  (A')  its  pro- 


91 

jected  horizontal  area  B  C  is  reduced  (B  C"),  and  that  if  we  consider 
the  width  of  the  plane  as  a  radius,  the  elevation  of  the  front  edge 
will  reduce  its  projected  horizontal  area  just  in  the  proportion  that 
the  versed  sine  C  D  is  increased  (C'D).  But,  as  for  the  sharpest 
practical  angle  of  flight  this  reduction  is  about  2  per  cent,  while  for 
the  lower  and  more  practical  angles  the  reduction  is  considerably 
less  than  1  per  cent,  this  factor  is  so  small  that  it  may  not  be 
considered  at  all  in  practical  flight." 

Now,  as  in  von  Loessel's  formulse  the  lift  is  proportional  to  the 
product  of  the  sine  and  cosine  of  the  angle  of  incidence,  and  this 
product  is  a  little  smaller  than  the  sine  of  the  same  angle,  and  the 
difference  increases  as  the  angle  increases,  we  see  that  the  small 
factor  mentioned  by  Sir  Maxim  is  taken  into  consideration  by  von 
Loessel,  and  the  formula  generalized  for  all  angles. 

This  careful  study  of  details  by  von  Loessel  explains  the  reason 
why  Sir  Maxim  so  highly  commends  his  work. 

Evidently,  the  lift  is  the  weight  that  can  be  raised  by  the  plane, 


138  APPENDIX 

and  the  drift  the  resistance  that  must  be  overcome  to  obtain 
forward  motion.  If  we,  therefore,  call  W  the  weight  of  the  plane 
and  substitute  it  for  L  in  the  formula  giving  the  lift,  we  will 
have: 

W  =  K  S  F2  sin  a  cos  a 

And  if  we  want  to  express  the  drift  in  H  P,  we  will  obtain  the  fol- 
lowing formula: 

HP  =  KSV*  sin2  a 
75 

because  the  formula  of  the  drift, 

D  =  K  S  V2  sin2  a 

gives  us  the  drift  in  kilograms  and  to  express  it  in  H  P,  we  must 
multiply  it  by  the  velocity,  F,  and  divide  by  75,  as  75  kilogram- 
meters  make  one  H.  P. 

By  dividing  the  L  or  W  by  the  H  P,  we  will  obtain  the  lift  or 
weight  per  H  P,  that  is, 

W       K  S  V2  sin  a  cos  a  _  K  S  V2  sin  a  cos  a  75  _  cos  a  75  _ 
HP  =      K  S  F3  sin2  a  K  S  F3  sin2  a          =  V  sin  a 

75 

_  75     cos  a  _  75  sin  a  _  75  75 

F      sin  a       F    cos  a       F  F  tg  a 

The  formula  giving  the  lift  and  the  formula  of  the  drift  expressed 
in  H  P  enable  us  to  calculate  any  one  of  the  elements  entering  in 
the  consideration  of  horizontal  flight,  when  we  assume  as  known 
the  other  elements.  But  these  formulae  are  not  final.  Other  and 
important  considerations  will  modify  them. 

So  far,  we  have  been  figuring  on  plane  surfaces,  but  it  is  a  well- 
known  fact  that  arched  surfaces  possess  greater  lifting  power,  for 
the  same  amount  of  energy  used,  than  plane  ones.  We  must, 
therefore,  know  the  coefficient  of  resistance  of  the  arched  surfaces, 
and  multiply  by  it  the  value  given  by  our  formulae.  As  this  co- 
efficient is  not  constant,  but  varies  with  the  arching  of  the  surface, 
we  must  determine  it  for  the  special  form  we  want  to  use,  and 
apply  the  value  found  to  the  formulae  giving  the  lift  and  drift.  If 


APPENDIX  139 

we  call  C  this  coefficient  of  curvature,  our  formulae  will  become: 

W  =  C  K  S  F2  sin  a  cos  a 
C  K  S  F3  sin*  a 


HP  = 


75 


in  which  C  is  greater  than  1. 

Another  consideration,  which  may  be  made  to  further  alter  these 
formulae,  is  the  knowledge  that  the  area  of  the  plane,  found  by 
multiplying  its  dimensions,  is  not  really  the  effective  area;  that  is, 
considering  the  actual  surface  area,  we  get  less  lifting  power  than 
we  ought  to.  A  plane  one  meter  square  will  not  lift  one-  tenth  as 
much  as  one  that  is  one  meter  wide  and  ten  meters  long.  This  is 
because  the  air  slips  off  at  the  ends.  In  designing  planes,  therefore, 
we  must  not  forget  that  area  alone  is  not  sufficient.  The  plane 
must  have  a  certain  length  of  entering  edge  in  proportion  to  its 
width. 

But  although  we  know  this  to  be  a  fact,  no  formula  seems  to  be 
reliable  enough  at  the  present  day  to  deserve  any  serious  consid- 
eration. We  can,  therefore,  do  away  with  them  all,  and  leave  the 
formulae  as  they  are. 

Let  us  consider,  now,  the  formula  giving  the  H  P. 

This  formula  gives  us  the  means  to  determine  the  theoretical 
resistance  to  the  forward  motion  of  the  plane.  But  in  practice 
we  know  that  using  a  motor  of  a  given  H  P  to  drive  a  propeller 
in  order  to  obtain  this  forward  motion,  the  propeller  does  not  de- 
liver all  of  the  power  transmitted  by  the  motor,  but  only  a  certain 
percentage,  according  to  the  efficiency  of  the  propeller  used.  The 
theoretical  power,  then,  given  by  our  formula,  is  cut  down  ac- 
cordingly. 

Tjl 

If  we  call  —  •  the  efficiency  of  the  propeller,  we  will  have  to 
10U 

multiply  by  it  the  theoretical  power  given  by  our  formula,  and 
we  will  have,  then: 


75  100 

Or,  if  we  want  to  know  what  must  be  the  H  P  of  the  motor  to  use, 
so  that,  when  reduced  by  the  slip  of  the  propeller,  it  will  give  us 


140  APPENDIX 

the  actual  power  required  to  drive  our  plane,  we  must  divide  the 
theoretical  power  by  the  efficiency  of  the  propeller,  and  in  this 
case  the  formula  will  be: 


A  H  P  =  •         -  — 

75  *100~  75  *1? 

But  this  is  not  all.  Besides  the  loss  through  the  slip  of  the  pro- 
peller, we  have  to  consider  the  head  resistance  of  the  framework 
of  a  flying  machine,  motor  and  aviator.  And  this  is  by  no  means 
a  matter  of  little  importance  or  easy  calculation.  It  all  depends 
on  the  shape,  cross  section  and  inclination  of  the  different  parts 
used  in  building  the  machine.  It  is,  therefore,  impossible  to  give 
specific  rules  for  the  computation  of  the  head  resistance.  All  we 
can  say  is  that  it  must  be  calculated  separately  for  every  machine, 
and  that  to  be  reduced  to  the  least,  we  must  avoid  plane  surfaces 
perpendicular  to  the  direction  of  motion  of  a  flying  machine,  and 
use  in  the  construction  of  the  framework  those  shapes  best  cal- 
culated to  reduce  head  resistance.  For  each  shape  and  cross  sec- 
tion of  bar,  there  is  a  special  coefficient,  which  must  be  applied  for 
each  special  case.  Round  bars,  for  instance,  or  oval,  bipointed 
bars,  set  with  one  of  the  points  to  the  direction  of  motion,  offer 
less  resistance  than  others  differently  shaped. 

Anyhow,  whatever  this  head  resistance  may  be,  the  power  neces- 
sary to  overcome  it  must  be  found  out  and  added  to  that  already 
calculated,  and  in  this  way  we  will  get  the  sum  total  of  the  actual 
power  required  to  obtain  horizontal  flight. 

In  other  words,  after  we  have  calculated  the  theoretical  power 
required  to  drive  our  plane  through  the  air,  we  must  find  out  the 
loss  of  power  through  the  propeller  slip  and  head  resistance,  and 
increase  accordingly  the  H  P  requirecl. 

If  we  suppose  to  have  calculated  the  surface  area  of  the  head 
resistance,  Sh,  and  consider  it  as  a  plane  surface  moving  perpen- 
dicularly in  the  direction  of  motion  of  the  flying  machine,  we  will 
have  the  formula: 

Rh=KSh  V* 
or  expressed  in  H  P: 


APPENDIX  141 

which  must  be  added  to  the  actual  H  P  found  before,  that  is, 
C  KSV3  sin2  a     100 
75  ^   E 

to  obtain  the  total  H  P  required  to  accomplish  horizontal  flight, 
so  that  the  total  H  P  will  be: 

TRp      (7  #  S  73  sin'  a      100      KShV 


Considering  the  expression  of  the  formulae  giving  the  drift  and 
lift,  we  see  that  the  drift  increases  as  the  square  of  the  sine  of  the 
angle  of  incidence,  and  the  lift  increases  as  the  product  of  the  sine 
and  cosine.  This  last  product  is  a  maximum  when  the  angle  is 
45°,  but  taking  into  account  the  drift,  we  find  that  the  best  lift 
drift  ratio  is  really  attained  for  angles  smaller  than  45°.  At  45°, 
as  sin  =  cos,  sin  X  cos  =  sin2,  and,  therefore,  L  =  D. 

As  the  practical  angles  of  flight  are  small,  it  follows  that  the  drift 
is  much  smaller  than  the  lift;  and  as  the  lift  is  in  reality  the  weight 
of  the  aeroplane,  or,  in  other  words,  the  force  of  gravity,  we  see 
that  the  power  required  to  raise  an  aeroplane  is  much  smaller  than 
the  force  of  gravity.  The  aeroplane,  therefore,  is  very  economical 
in  regard  to  power,  compared  with  the  helicopter  or  ornithopter, 
as  these  machines,  to  leave  the  ground,  must  produce  first  of  all 
a  vertical  force  powerful  enough  to  overcome  that  of  gravity,  and 
this  without  considering  the  power  lost  in  consequence  of  the  ex- 
treme fluidity  of  the  air. 

Sir  Maxim  in  this  regard  says: 

"Recently  there  has  been  a  great  deal  of  discussion  regarding  the 
comparative  merits  of  the  aeroplane  system  and  the  helicopter. 
Some  condemn  both  systems  and  pin  their  faith  to  flapping  wings. 
It  has  been  contended  that  the  screw  propeller  is  extremely  wasteful 
in  energy,  and  that  in  all  nature  neither  fish  nor  bird  propels  itself 
by  means  of  a  screw.  As  we  do  not  find  a  screw  in  nature,  why 
then  should  we  employ  it  in  a  machine  for  performing  artificial 
flight?  Why  not  stick  to  nature?  In  reply  to  this,  I  would  say 
that  even  nature  has  her  limits,  beyond  which  she  cannot  go. 
When  a  boy  was  told  that  everything  was  possible  with  God,  he 
asked:  'Could  God  make  a  two-year  old  calf  in  five  minutes?' 
He  was  told  that  God  certainly  could.  'But,'  said  the  boy  'would 


142  APPENDIX 

the  calf  be  two  years  old? '  It  appears  to  me  that  there  is  nothing 
in  nature  which  is  more  efficient,  or  gets  a  better  grip  on  the  water 
than  a  well-made  screw  propeller,  and  no  doubt  there  would  have 
been  fish  with  screw  propellers,  provided  that  Dame  Nature  could 
have  made  an  animal  in  two  pieces.  It  is  very  evident  that  no 
living  creature  could  be  made  in  two  pieces,  and  two  pieces  are 
necessary  if  one  part  is  stationary  and  the  other  revolves;  however, 
the  tail  and  fins  very  often  approximate  to  the  action  of  the  pro- 
peller blades;  they  turn  first  to  the  right  and  then  to  the  left,  pro- 
ducing a  sculling  effect  which  is  practically  the  same.  This  argu- 
ment might  also  be  used  against  locomotives.  In  all  nature,  we 
do  not  find  an  animal  traveling  on  wheels,  but  it  is  quite  possible 
that  a  locomotive  might  be  made  that  would  walk  on  legs  at  the 
rate  of  two  or  three  miles  per  hour.  But  locomotives  with  wheels 
are  able  to  travel  at  least  three  times  as  fast  as  the  fleetest  animal 
with  legs,  and  to  continue  doing  so  for  many  hours  at  a  time,  even 
when  attached  to  a  very  heavy  load.  In  order  to  build  a  flying 
machine  with  flapping  wings,  to  exactly  imitate  birds,  a  very  com- 
plicated system  of  levers,  cams,  cranks,  etc.,  would  have  to  be 
employed  and  these  of  themselves  would  weigh  more  than  the 
wings  would  be  able  to  lift." 

In  order  to  apply  to  a  practical  case  the  formula  given,  let  us 
suppose  that  we  want  to  build  an  aeroplane. 

We  must  assume  the  knowledge  of  some  of  the  values  of  the 
formula  to  calculate  the  other  values. 

In  the  formula  giving  the  lift: 

W  =  C  K  S  Vz  sin  a  cos  a 

we  know  only  the  value  of  the  coefficient  K,  and  we  may  know 
that  of  C,  if  we  give  to  our  planes  a  curvature  whose  coefficient  is 
known,  otherwise  we  have  to  find  it  out  practically  ourselves.  Sup- 
posing, then,  that  we  have  the  value  of  C,  we  need  to  know  at  least 
three  more  values,  before  we  can  determine  the  fourth. 

Now,  we  usually  know  the  weight  of  our  machine  and  the  angle 
of  incidence,  which  we  choose  as  best;  what  we  must  determine, 
therefore,  is  either  the  surface,  to  find  out  the  velocity,  or  the 
velocity,  to  figure  out  the  surface. 

It  is  up  to  us,  then,  to  set  out  the  value  of  the  one  or  the  other, 
according  to  our  own  wish  in  regard  to  the  speed  of  the  machine. 


APPENDIX  143 

If  we  increase  the  speed,  we  diminish  the  surface,  and  vice  versa. 
But,  of  course,  this  is  by  no  means  arbitrary,  and  we  must  have 
an  idea  right  from  the  beginning,  of  what  we  want,  otherwise  we 
will  get  the  data  of  a  machine  which  theoretically  fulfills  our  wants, 
but  practically  is  an  impossibility. 

If  we  started  to  figure  out  the  weight  of  the  machine,  we  must 
have  considered  its  dimensions  in  regard  to  its  approximate  sur- 
face, so  as  not  to  compel  us  at  the  end  to  alter  completely  its  di- 
mensions and  consequently  its  weight.  So  it  is,  too,  in  regard  to 
the  velocity  and  load,  as  we  must  figure  on  the  strength  of  the 
material  in  regard  to  the  stress  that  the  framework  can  stand  with- 
out a  breakdown.  The  same  rule  holds  in  regard  to  the  proportion 
of  the  weight  to  the  power  of  the  motor. 

If  we  have  started  without  an  approximately  correct  idea  of  what 
our  machine  is  going  to  be,  we  might  find  out  at  the  end  that  either 
the  surface  is  too  small,  or  the  machine  is  too  weak,  or  the  motor 
is  too  heavy  for  the  power  it  gives. 

In  this  case,  we  would  be  in  the  same  position  of  the  early  ex- 
perimenters, who  had  to  spend  years  before  they  could  find  out 
the  proper  data  to  build  a  successful  machine. 

The  best  thing  to  do,  then,  is  to  study  the  approved  types  of 
existing  machines,  and  vary  their  proportions  according  to  our 
special  case. 

Anyhow,  more  than  one  calculation  is  always  necessary  to 
vary  our  erroneous  assumptions,  before  we  get  the  final  correct 
result. 

Let  us  see,  now,  how  from  our  fundamental  formula  giving  the 
W,  we  can  derive  the  value  of  all  the  other  unknown  quantities 
in  it  contained. 

From  the  formula: 

W  =  C  KSV*  sin  a  cos  a 
we  have: 

W 


S  = 
V  ^ 


C  K  V2  sin  a  cos  a 

W 

\/C  K  S  sin  a  cos  a 

W 
C  KS  V* 


sin  a  cos  a  = 


144  APPENDIX 

For  the  last  formula,  as  we  have  no  tables  giving  the  product 
of  sines  by  cosines,  we  will  have  to  use  the  knowledge  that  the  sine 
by  the  cosine  of  an  angle  is  equal  to  one-half  the  sine  of  an  angle 
double.  Therefore,  after  we  find  the  product  sin  by  cos,  we  have 
to  double  it,  to  know  the  sine  of  the  angle  double;  then  we  find, 
in  the  tables  of  the  natural  functions  of  angles,  which  angle  in 
degrees  corresponds  to  this  sine,  and  we  divide  the  degree  by  two, 
to  arrive  at  the  angle  we  are  looking  for.  For  this  reason,  the 
formula  may  be  expressed : 

2W 

sm  2a  =  cl^sY* 

and,  if  found  convenient,  the  same  substitution  may  be  made  in 
all  the  other  formulae. 

Suppose,  now,  that  for  our  machine  we  want  to  use  a  30  H.  P. 
motor  weighing  65  Kgs.,  that  the  plane  surface  is  25  square  meters 
and  inclined  at  an  angle  of  7°,  that  the  framework  weighs  125  Kgs. 
according  to  our  calculations  in  regard  to  the  strength  of  the  ma- 
terial chosen,  that  the  coefficient  of  curvature  is  1.25,  and,  finally, 
that  the  aviator  weighs  60  Kgs.  The  total  weight  of  the  machine 
will  be,  then:  65  +  125  +  60  =  250  Kgs. 

Knowing  the  weight,  the  surface  and  the  angle  of  incidence,  we 
can  find  the  velocity  from  the  formula: 


w 
V  =  '  ' 


C  K  S  sin  a  cos  a 

as  we  have  all  the  numeric  values  of  the  quantities  which  deter- 
mine the  value  of  V;  that  is,  W  =  250,  C  =  1.25,  K  =  0.080, 
S  =  25,  sin  angle  7°  =  0.1219  and  cos  angle  7°  =  0.9925.  There- 
fore: 


V 


28 


1.25  X  0.080  X  25  X  0.1219  X  0.9925 

The  velocity  of  our  aeroplane  will  be,  then,  28  meters  per  second. 
Found  the  value  of  V,  we  can  find  the  H  P,  that  is : 


75  75 


APPENDIX  145 

Supposing  that  the  efficiency  of  the  propeller  we  want  to  use  is 

80 

—  ,  the  actual  H  P  will  be: 
100 

-j  /-,  p 

A  H  P  =  10.82  X  —  =  13.52 
80 

If  we  consider  the  surface  area  of  the  head  resistance  as  equiva- 
lent to  a  plane  of  0.50  square  meters  moving  orthogonally  through 
the  air,  then: 

3  j?  =  U70  H  p 


Attention  should  be  paid  to  the  fact  that  only  one-half  square 
meter  of  surface  of  head  resistance  requires  about  as  much  power 
as  25  square  meters,  or  50  times  as  much  surface,  set  at  the  angle 
of  7°.  This  tells  clearly  the  enormous  power  required  to  drive  the 
framework  alone  of  the  machine. 

The  total  H  P  will  be,  therefore: 

T  H  P  =  13.52  +  11.70  =  25.22 

As  our  motor  is  30  H  P,  we  have  a  good  margin  left,  and,  con- 
sequently, the  machine  will  fly. 

And  now  that  our  aeroplane  is  ready  to  take  the  air,  let  me  jump 
in  and  fly  away.* 

*  The  formula  giving  the  TUP  could  be  simplified  into 
K  V  3  (100  C  S  sin  2  a  +  Sh  E) 

~7^E~ 

but  it  was  not,  in  order  to  show  all  the  different  data  entering  in  the 
final  calculation. 

The  aerodynamical  formula  given  in  the  metric  system  may  be  re- 
duced in  British  units  by  making:  K  =  0.003,  S  =  area  in  square  feet, 
V  —  velocity  in  miles  per  hour,  W  =  pounds  and  H  P=  33,000  foot 
pounds  per  minute.  Thus,  the  formula  visibly  changed  would  be 
those  in  which  enters  the  computation  of  the  horsepower,  that  is, 


33,000 

and  following;  the  others  remaining  the  same  algebraically  and  differing 
arithmetically  only  relatively,  according  to  the  values  substituted  for 
the  letters  in  the  application  of  either  the  metric  or  the  British  system 
of  measurement,  but  being  the  same  in  absolute  value. 


DEFINITIONS 

Acetone  is  a  limpid,  colorless,  volatile  liquid  of  penetrating 
ethereal  odor  and  pungent  taste,  obtained  from  the  products  of 
the  destructive  distillation  of  wood  or  by  heating  calcium  acetate. 
It  is  a  useful  solvent  for  gums,  resins  and  fats. 

Aerodrome  is  the  ground  used  for  the  practice  of  aviation. 

Aeroplane  is  a  power-driven  aircraft  sustained  in  flight  by  the 
reaction  of  the  air  against  wings  set  at  an  angle  with  the  line  of 
motion.  It  is  distinguished  as  monoplane  and  multiplane,  accord- 
ing to  the  number  of  superposed  wings  used;  the  biplane,  triplane, 
etc.,  being  particular  cases  of  the  multiplane. 

Aileron  is  a  controlling  plane  hinged  horizontally  to  the  rear  of 
a  wing  toward  the  tip  and  used  for  lateral  control. 

Air  speed  meter  is  an  instrument  which  measures  the  velocity 
of  the  air  and,  as  a  consequence,  that  of  an  aeroplane  when  installed 
on  it. 

Algebra  is  the  science  of  numbers  expressed  by  letters  and 
symbols. 

Equation  is  the  expression  of  the  condition  of  equality  between 
two  algebraic  quantities  or  set  of  quantities,  the  sign  =  being 
placed  between  them. 

Formula  is  a  rule  or  principle  expressed  in  algebraic  lan- 
guage. 

From  the  above  definitions,  it  follows  that  the  aerodynamical 
formulae  are  equations.  What  we  want  to  know,  then,  is  how  to 
solve  an  equation,  in  order  to  find  out  the  values  of  the  different 
quantities  expressed  by  the  letters  of  the  formulae. 

In  arithmetic,  we  express  quantities  by  means  of  numbers;  in 
algebra,  we  use  letters,  which  give  us  a  better  opportunity  to  gen- 
eralize a  given  rule.  Suppose,  for  instance,  that  we  want  to  express 
in  a  general  way  how  to  find  the  surface  area  of  a  rectangle.  Geom- 
etry teaches  us  that  this  is  accomplished  by  multiplying  one  of  its 
dimensions  by  the  other.  To  solve  the  problem  arithmetically, 
then,  we  have  to  know  the  value  of  the  two  dimensions  expressed 

147 


148  DEFINITIONS 

in  numbers,  and  if  these  two  numbers  are  5  and  2,  we  will  multiply 
one  by  the  other  and  say: 

5  X  2  =  10 

Algebraically,  instead,  we  do  not  need  to  know  these  numeric 
values,  but  we  will  call  one  of  the  dimensions  A,  for  instance,  the 
other  B  and  the  surface  area  S,  and  we  will  say  that 

S  =  A  X  B,  or  simply:  S  =  A  B, 

as,  in  algebra,  the  absence  of  a  sign  between  letters  or  one  number 
and  letters  means  multiplication. 

This  is  the  general  way  we  express  in  our  case  the  surface  area 
of  a  rectangle,  and  we  call  it  a  formula.  To  solve  this  formula  or 
equation  arithmetically,  we  have  to  know  the  numeric  values  of 
A  and  B,  substitute  them  respectively  for  the  letters  and  multiply 
one  by  the  other.  If  A  =  6,  B  =  3,  then: 

S  =  6  X  3  =  18 

Analyzing  what  we  have  done,  we  see  that  to  find  the  value  of 
S,  we  need  to  know  the  value  of  A  and  B,  that  is,  out  of  three 
quantities  of  a  formula,  we  must  know  two  to  find  out  the  third. 
Generalizing  this  special  case,  we  will  say  that  in  order  to  find  the 
value  of  one  quantity  of  a  formula,  we  must  know  the  value  of 
all  the  other  quantities. 

To  solve  an  equation,  we  must  know  the  following  rules: 

We  can  add  or  subtract,  multiply  or  divide  by,  the  same  number 
both  members  of  an  equation  without  altering  the  equality. 

Let  us  explain  this  arithmetically,  first.    If  we  have  the  expression : 

5+4+1 =6+2+2 

or,  which  is  the  same: 

10  =  10 

we  can  add  to  each  member  the  same  number,  say,  5,  without 
altering  the  equality;  in  fact,  it  will  be: 

10  +  5  =  10  +  5 

In  the  same  way,  we  can  subtract  the  number  3,  and  we  will  have: 
10  —  3  =  10  —  3 


DEFINITIONS  149 

Or,  multiplying  by  4: 

10  X  4  =  10  X  4 

And,  finally,  dividing  by  2: 

10  _  10 
2    ~=~2 

In  the  same  way  we  can  raise  to  power,  or  extract  the  root  of, 
both  members  of  an  equation  without  altering  its  equality,  as  these 
two  last  cases  really  amount  to  special  instances  of  multiplication 
and  division.  So,  it  will  be: 

10  =  10;  102  =  102,  that  is,  10  X  10  =  10  X  10. 
16  =  16;  \/16  =  A/16,  that  is,  4=4. 

Using  the  same  process  algebraically,  we  will  have  the  following 
equations: 

If  a  =  a 

adding  6,  a  +  b  =  a  +  b 

subtracting  c,  a  —  c  =  a  —  c 

multiplying  by  d,  a  d  =  a  d 

dividing  by  e,  a  _  a 

e      e 

raising  to  power,      a2  =  a2,        a3  =  a3,        an  =  an 
extracting  the  root,  \/a  =  \/a,       \/a  =  -\/a,       -\/a  =  \/a 
Suppose  now  that  we  have  this  equation : 
a  —  b  +  c  =  d  +  e 

and  that  we  add  the  quantity  b  to  both  members,  that  is: 
a  -  b  +  c  +b  =  d  +  e  +b 

In  the  first  member  of  this  equation,  we  see  that  there  is  the  same 
quantity,  b,  added  and  subtracted  in  the  meantime.  As  the  result 
would  be  zero  (+6  —  6  =  0),  we  can  suppress  it  and  have: 

a  +  c  =  d  +  e  +  6 

Comparing  this  equation  with  the  first  one,  we  see  that  the  term  6, 
which  was  in  the  first  member  with  a  negative  sign  (--),  passed 
to  the  second  member  with  a  positive  sign  (+). 


150  DEFINITIONS 

If  we  now  subtract  from  both  members  of  the  equation 

a  —  b  +  c  =  d  +  e 
the  quantity  c,  we  will  have: 

a  -  b  +  c  —  c  =  d  +  e  —  c 
As  +  c  —  c  =  0,  it  will  be: 

a  —  b  =  d  -\-  e  —  c 

that  is,  +  c  in  the  first  member  became  —  c  in  the  second. 
If  we  have  the  equation: 


and  multiply  all  by  d,  we  will  have: 

a  b  d  =  -  d 
d 


or 


d 

aba  = c- 
d 


-  =  1,  therefore: 
d 


a  b  d  =  c  X  1,  that  is:  a  b  d  =  c 

We  see,  then,  that  the  quantity  d,  which  was  in  the  second 
member  as  a  divisor,  passed  to  the  first  member  as  a  multiplicator. 
And,  finally,  if  we  have  the  equation: 

a  b  =  c 
and  divide  all  by  b,  we  have: 

a  b      c  be  c  c 

_..,     .....     oxl__,     a  =  . 

That  is,  b  from  multiplicator  in  the  first  member  became  divisor 
in  the  second. 

We  will  say,  therefore,  that  we  can  pass  one  term  from  one 
member  of  an  equation  to  the  other  by  changing  its  sign  without 
altering  the  equality. 

We  understand,  now,  why  from  our  aerodynamical  formula: 

W  =  C  K  S  V2  sin  a  cos  a 


DEFINITIONS  151 

we  obtain  the  values  of  each  quantity  by  passing  the  others  from 
multiplicators  in  one  member  to  divisors  in  the  other,  and  in  the 
case  of  the  velocity  by  extracting  the  square  root  from  both  mem- 
bers. 

Altimeter  is  a  modified  barometer  used  for  measuring  height. 

Anemometer  is  an  instrument  for  measuring  the  force  and 
velocity  of  the  wind. 

Angle  of  attack  is  the  angle  formed  by  the  chord  of  the  wings 
with  the  line  of  flight  when  the  aeroplane  climbs  or  descends. 

Angle  of  sweepback  is  the  angle  formed  by  the  leading  edge 
of  a  wing  with  the  lateral  axis  of  an  aeroplane.  Best  climbing 
angle  is  approximately  halfway  between  the  maximum  and  optimum 
angles. 

Flying  angle  of  incidence  is  the  angle  formed  by  the  neutral  line 
of  a  plane  with  the  line  of  flight.  It  is  positive  when  formed  above 
the  line  of  flight;  negative,  when  formed  below  the  line  of  flight; 
zero,  when  the  neutral  line  is  parallel  with  the  line  of  flight. 

Gliding  angle  is  the  angle  of  attack  of  an  aeroplane  descending 
by  force  of  gravity. 

Lateral  dihedral  angle  is  the  angle  formed  by  two  wings  when 
they  are  tipped  upward. 

Longitudinal  dihedral  angle  is  the  angle  formed  by  the  pro- 
longation of  the  chord  of  the  wings  with  the  prolongation  of  the 
chord  of  the  horizontal  stabilizer.  If  the  angle  is  formed  by  the 
prolongation  of  the  neutral  lines,  it  is  the  flying  longitudinal  dihedral 
angle;  if  formed  by  the  prolongation  of  the  chords,  it  is  the  rigger's 
longitudinal  dihedral  angle.  Rigger's  angle  of  incidence  is  the  angle 
formed  by  the  chord  of  a  plane  with  the  line  of  thrust. 

Maximum  angle  of  incidence  is  the  greatest  angle  at  which,  with 
a  given  power,  surface  and  weight,  horizontal  flight  can  be  main- 
tained. 

Minimum  angle  of  incidence  is  the  smallest  angle  at  which,  with 
a  given  power,  surface  and  weight,  horizontal  flight  can  be  main- 
tained. 

Optimum  angle  of  incidence  is  the  angle  at  which  the  lift  drift 
ratio  is  the  best. 

Antidrift  wire  is  a  wire  which  acts  against  the  tension  of  a  drift 
wire.  There  are  two  kinds  of  antidrift  wires:  center  section  and 
frame  antidrift  wires. 


152  DEFINITIONS 

Aspect  ratio  is  the  proportion  of  the  span  to  the  chord  of  a  plane. 

Aviation  is  the  branch  of  aeronautics  that  treats  of  the  gasless 
aircraft. 

Banana  oil  or  varnish  is  a  mixture  of  acetone  and  amylacetate 
with  liquid  celluloid,  having  a  marked  banana-like  odor. 

Bank  is  to  tilt  an  aeroplane  sideways  when  turning. 

Barograph  is  a  self-registering  barometer,  which  gives  a  con- 
tinuous graphic  record  of  the  fluctuations  of  the  atmospheric 
pressure. 

Barometer  is  an  instrument  which  measures  the  pressure  of  the 
atmosphere. 

Bay  is  a  compartment  in  the  fuselage  or  in  the  wings  of  a  multi- 
plane. 

Bending  is  the  combination  of  the  compression  and  tension 


Blow  torch  is  a  lamp  burning  gasoline,  forced  by  air  pressure 
through  a  hot,  holed  tube,  to  gasify  and  mix  it  with  air  and  pro- 
duce an  intensely  hot,  blue  flame. 

Cabane  is  a  metallic  framework  to  which  are  attached  the  land- 
ing wires  of  the  wings  of  a  monoplane  or  the  upper  wings  of  a  multi- 
plane that  has  no  center  section. 

Camber  is  the  curvature  of  a  plane. 

Cap  strip  is  the  flange  of  a  rib. 

Castellated  nut  is  a  nut  with  grooves  in  its  upper  face  to  receive 
a  cotter  pin. 

Cavitation  is  the  rarefaction  of  air  produced  in  the  space  imme- 
diately in  the  rear  of  swiftly  revolving  propeller  blades,  due  to  the 
rapid  cleavage  of  the  air  by  the  blades  and  the  relatively  slow 
action  of  the  air  in  closing  in  behind  the  moving  blades. 

Cellulose  is  the  principal  component  of  all  vegetable  tissues. 
Cotton  and  filter  paper  are  almost  pure  cellulose. 

Center  line  of  pressure  is  a  line  running  from  tip  to  tip  of  a  wing 
and  through  which  all  the  air  forces  acting  on  the  wing  may  be 
said  to  act. 

Center  of  gravity  of  a  body  is  the  point  about  which  all  its  parts 
are  balanced. 

Center  of  lift  is  the  point  of  application  of  the  resultant  of  all 
the  lifting  forces  of  an  aeroplane. 

Center  of  pressure  is  the  point  at  which  the  whole  amount  of 


DEFINITIONS  153 

pressure  may  be  concentrated  with  the  same  effect  as  when  dis- 
tributed. 

Center  of  resistance  is  the  point  of  application  of  the  resultant 
of  all  the  forces  of  the  passive  drift  acting  against  the  different 
parts  of  an  aeroplane. 

Center  of  thrust  is  the  point  of  application  of  the  thrust  of  the 
propeller. 

Center  section  is  the  central  structure  which  connects  the  upper 
wings  of  a  multiplane. 

Centrifugal  force  is  the  reaction  of  a  body  against  a  force  that 
is  causing  it  to  move  in  a  curved  path. 

Centripetal  force  is  a  force  drawing  a  body  toward  the  center 
around  which  it  revolves. 

Chord  is  the  straight  line  drawn  from  the  leading  to  the  trailing 
edge  of  a  plane. 

Clevis  pin  is  a  rivet  with  a  hole  in  the  point  for  the  passage  of  a 
cotter  pin. 

Cockpit  is  the  compartment  of  the  fuselage  which  contains  the 
pilot's  seat. 

Compass  is  an  instrument  by  means  of  which  the  directive  mag- 
netic force  of  the  earth  upon  a  freely  suspended  magnetic  needle 
is  utilized  to  determine  horizontal  directions  in  reference  to  the 
north  and  the  other  cardinal  points. 

Compression  is  the  stress  which  tends  to  crush  a  body. 

Control  lever  is  a  wooden  stick  or  metallic  tube  to  which  are 
attached  the  cables  of  the  ailerons  and  elevators  for  controlling 
their  motions. 

Controlling  plane  is  a  plane  designed  to  control  mechanically 
the  motions  of  an  aeroplane  longitudinally,  laterally  or  directionally. 
There  are  three  kinds  of  controlling  planes:  elevators,  ailerons  and 
rudder. 

Cotter  pin  is  a  split  key  made  by  bending  a  half  round  wire  with 
the  flat  face  inside,  so  as  to  form  an  eye  at  the  bend  and  bring 
together  the  two  halves  or  leaves,  which  thus  make  a  round  wire 
open  in  the  middle.  It  is  inserted  in  the  hole  of  a  clevis  pin  to  lock 
it  safely  in  place  by  spreading  out  the  leaves  or  to  lock  the  nut  of 
a  bolt  provided  with  an  apposite  hole  at  its  threaded  end. 

Decalage  is  the  difference  in  the  angle  of  incidence  of  any  two 
planes  in  the  same  machine. 


154  DEFINITIONS 

Disk  wheel  is  a  wheel  stream-lined  by  covering  its  spokes  with 
cone-shaped  sheets  of  metal,  celluloid  or  doped  fabric. 

Dope  is  a  solution  of  cellulose  nitrate  or  acetate  and  banana  oil 
used  to  paint  the  fabric  covering  of  aeroplanes  to  make  it  taut  and 
airproof. 

Dowel  is  a  round  stringer. 

Drift  is  the  horizontal  component  of  the  air  resistance  against 
a  plane. 

Active  drift  is  the  drift  of  the  lifting  planes. 

Passive  drift  is  the  drift  of  all  the  other  parts  of  an  ae'ro- 
plane. 

Total  drift  is  the  entire  resistance  of  the  air  against  the  machine 
in  flight  and  includes  the  active  drift,  the  passive  drift  and  the 
skin  friction. 

Drift  meter  is  an  instrument  which  indicates  the  leeway  of  an 
aeroplane.  It  consists  of  a  telescope,  containing  a  series  of  parallel 
hairs  with  a  graduate  scale  and  pointer,  mounted  vertically  to 
enable  the  pilot  to  look  down  upon  the  ground  directly  beneath 
him.  By  turning  the  telescope  so  that  the  hairs  are  parallel  with 
the  line  of  motion,  indicated  by  roads,  rivers  or  other  landmarks, 
the  exact  leeway  or  drift  of  the  aeroplane  is  measured  by  the  needle 
of  the  indicator. 

Drift  wire  is  a  wire  which  supports  some  part  of  a  machine  against 
the  drift  during  flight.  There  are  three  kinds  of  drift  wires:  the 
wing  drift  wires,  which  run  from  the  nose  plate  to  the  wings;  the 
center  section  drift  wires,  which  go  from  the  upper  longerons  to 
the  front  struts  of  the  center  section;  and  the  frame  drift  wires, 
which  are  attached  between  compression  ribs  inside  of  the  frame 
and  are  hidden  from  view  by  the  fabric  covering. 

Droop  is  the  increase  in  the  angle  of  incidence  of  the  ailerons  and 
elevators  to  compensate  the  decrease  which  occurs  in  flight  owing 
to  the  resistance  of  the  air. 

Eddy  is  a  current  of  air  moving  in  a  direction  contrary  to  the 
main  current. 

Efficiency  of  construction  is  the  ratio  of  the  lifting  surface  to 
the  passive  drift  surface  of  an  aeroplane.  (If  the  lifting  surface  is 
200  square  feet  and  the  passive  drift  surface  is  10  square  feet,  the 
efficiency  of  construction  is  200  :  10  =  20.) 

Elevator  is  a  controlling  plane  hinged  horizontally  to  the  rear 


DEFINITIONS  155 

of  the  horizontal  stabilizer  of  a  machine  to  direct  it  upwards  or 
downwards. 

Empennage  is  the  tail  of  an  aeroplane. 

Equilibrium  is  the  state  of  balance  produced  by  the  mutual 
counter  action  of  two  or  more  forces.  Equilibrium  is  characterized 
by  three  phases:  stable,  unstable  and  indifferent  or  neutral.  A 
body  is  in  a  state  of  stable  equilibrium  when,  being  disturbed,  it 
tends  to  return  to  its  previous  position;  in  this  state,  the  center  of 
gravity  of  the  body  is  in  its  lowest  possible  place.  A  body  is  in 
a  state  of  unstable  equilibrium  when,  being  disturbed,  it  tends  to 
move  away  from  its  previous  position;  in  this  state,  the  center  of 
gravity  of  the  body  is  in  its  highest  possible  place.  A  body  is  in 
a  state  of  indifferent  or  neutral  equilibrium  when  it  keeps  its  bal- 
ance independently  of  the  position  it  is  put  in;  in  this  state,  the 
center  of  gravity  of  the  body  is  at  its  center. 

Extension  or  overhang  is  the  lateral  extension  of  an  upper  wing 
beyond  the  span  of  a  lower  wing  of  a  multiplane. 

Factor  of  safety  is  the  ratio  of  the  stress  of  collapse  of  a  body  to 
the  maximum  stress  it  is  called  upon  to  withstand. 

Fair  lead  is  a  short  metallic  tube  with  funnel-shaped  ends  through 
which  runs  a  cable. 

Fairing  is  the  additional  material  used  to  stream-line  a  body. 

Ferrule  is  a  short  tubular  coupling  used  for  locking  a  solid  wire 
loop. 

Fineness  is  the  ratio  of  the  length  to  the  width  of  a  stream-lined 
body.  It  is  directly  proportional  to  the  velocity. 

Fitting  is  a  metallic  fixture  which  connects  the  joints  of  different 
pieces  of  the  framework  of  a  machine. 

Flying  boat  is  a  hydroaeroplane  with  a  hull  in  the  place  of  a 
fuselage. 

Flying  wire  is  a  wire  attached  to  a  point  of  a  wing  to  prevent  it 
from  breaking  when  the  machine  is  in  flight. 

Flux  is  a  substance  that  promotes  the  fusion  of  metals,  pre- 
vents their  oxidation  under  the  action  of  heat  and  cleans  their 
surface. 

Foot  rudder  bar  is  a  wooden  bar  to  which  are  attached  the  rudder 
control  wires. 

Fuselage  is  the  stream-lined  main  body  of  an  aeroplane  to  which 
all  the  other  parts  are  attached. 


156  DEFINITIONS 

Gap  is  the  space  between  two  planes,  measured  from  chord  to 
chord. 

Hangar  is  an  aeroplane  shed. 

Helicopter  is  a  machine  intended  to  fly  by  means  of  horizontal 
screw  propellers. 

Horizontal  equivalent  is  the  horizontal  projection  of  a  body. 

Horizontal  stabilizer  or  fixed  tail  plane  is  a  plane  bolted  on  the 
upper  tail  end  of  the  fuselage  to  give  inherent  longitudinal  stability 
to  an  aeroplane. 

Hydroaeroplane  is  a  machine  with  pontoons  attached  to  the 
undercarriage  to  enable  it  to  rest  on  and  rise  from  water. 

Hydrochloric  acid  or  muriatic  acid  is  a  colorless,  corrosive,  pun- 
gent gas,  exceedingly  soluble  in  water.  What  is  commonly  known 
as  hydrochloric  acid  is  a  strong  aqueous  solution,  colored  yellow 
by  impurities.  It  is  generally  made  by  the  action  of  strong  sul- 
phuric acid  on  common  salt. 

Hydroplane  is  a  flat  bottomed,  high-powered  motor  boat,  which 
skims  at  a  high  speed  on  the  surface  of  the  water. 

Inclinometer  is  a  curved  spirit  level  which  indicates  the  degree 
of  inclination  of  an  aeroplane  with  the  horizontal.  There  are  two 
kinds  of  inclinometers:  one  determines  the  angle  of  attack;  the 
other,  called  laterometer  or  bank  indicator,  the  angle  of  bank. 

Inertia  is  that  property  of  matter  by  virtue  of  which  it  persists 
in  its  state  of  rest  or  of  uniform  motion  in  a  straight  line  unless 
it  is  acted  upon  by  some  external  force. 

Interference  is  the  detrimental  effect  produced  in  the  gap  by 
the  rush  of  air  or  wash,  which  disturbs  both  the  rarefaction  of  the 
top  camber  of  the  lower  wing  and  the  compression  of  the  lower 
camber  of  the  upper  wing. 

Inspection  cover  is  an  accessible  stream-lined  cover,  fastened  to 
the  upper  longerons  by  hinges  on  each  side,  which  can  easily  be 
removed  to  inspect  the  control  and  fuselage  wires. 

Keel  surface  is  the  total  side  elevation  surface  of  a  machine. 
(Body,  wings,  struts,  wires,  wheels,  etc.) 

King  post  is  a  short  mast  bolted  to  a  plane  for  the  attachment 
of  cables  or  wires. 

Landing  wire  is  a  wire  attached  to  a  point  of  a  wing  to  prevent 
it  from  breaking  when  the  machine  lands  or  stands  on  the  ground 
or  when  the  wing  is  subjected  to  a  reversal  of  load. 


DEFINITIONS  157 

Leading  or  entering  edge  is  the  front  edge  of  a  plane. 

Level  is  an  instrument  used  to  determine  a  horizontal  line. 

Lift  of  an  inclined,  cambered  plane  is  the  vertical  component  of 
the  air  resistance  against  the  lower  camber,  enhanced  almost  to 
its  full  power  by  the  rarefaction  on  the  upper  camber,  which  facili- 
tates the  lifting  force,  owing  to  the  difference  in  the  density  of  the 
two  currents  of  air  flowing  along  the  surfaces  of  the  plane. 

Lift  drift  ratio  is  the  proportion  of  lift  to  drift.  Considering  the 
lifting  planes  alone,  it  is  the  ratio  of  lift  to  drift;  considering  the 
entire  machine,  it  is  the  ratio  of  lift  to  total  drift. 

Line  of  flight  is  the  direction  in  which  flight  takes  place.  It  is 
referred  to  the  horizontal,  and,  therefore,  the  line  of  flight  is  the 
horizontal. 

Longeron  is  a  long  wooden  piece  running  longitudinally  in  the 
fuselage. 

Loop  is  the  doubling  of  a  wire  in  such  a  way  as  to  form  an  eye. 

Margin  of  lift  is  the  height  to  which  an  aeroplane  can  rise  in  a 
given  time  and  from  a  given  altitude. 

Margin  of  power  is  the  power  available  above  that  necessary  to 
maintain  horizontal  flight. 

Metric  system  is  the  method  of  measurement  based  on  the  meter, 

which  theoretically  is  the part  of  the  quadrant  of  a 

10,000,000 

terrestrial  meridian,  and  actually  is  the  length  of  a  bar  of  platinum 
designed  to  represent  that  dimension. 

The  metric  system  removes  the  confusion  arising  out  of  the 
excessive  diversity  of  weights  and  measures  prevailing  in  the  world, 
by  substituting  in  place  of  the  arbitrary  and  inconsistent  systems 
actually  in  use,  a  single  one  constructed  on  scientific  principles  and 
resting  on  a  natural  and  invariable  standard. 

The  unit  of  length  is  the  meter  (30.37  inches) ;  the  unit  of  surface, 
the  square  meter  (1550  square  inches);  the  unit  of  capacity,  the 
liter  (1.0567  quarts);  and  the  unit  of  weight,  the  gram  (15.432 
grains  troy). 

Each  unit  has  its  decimal  multiples  and  submultiples;  that  is, 
weights  and  measures  ten  times  larger  or  ten  times  smaller  than 
the  unit  of  the  denomination  preceding. 

The  prefixes  denoting  multiples  are  derived  from  the  Greek  lan- 
guage, and  are:  deca,  ten;  hecto,  hundred;  kilo,  thousand;  and  myria, 


158  DEFINITIONS 

ten  thousand.  Those  denoting  submultiples  are  from  the  Latin 
and  are:  deci,  tenth;  centi,  hundredth,  and  milli,  thousandth. 

The  unit  of  itinerary  measure  is  the  kilometer  or  1000  meters 
(0.62138  mile),  and  the  unit  of  commercial  weight  is  the  kilogram 
or  1000  grams  (2.205  Ib.  avoirdupois). 

The  meter  is  divided  in  ten  decimeters,  the  decimeter  in  ten 
centimeters,  the  centimeter  in  ten  millimeters;  just  as  the  dollar 
is  divided  in  ten  dimes,  the  dime  in  ten  cents,  the  cent  in  ten  mills; 
so  that  it  is  just  as  easy  to  figure  out  in  meters  as  it  is  in  dollars. 

The  abbreviation  of  kilometer  is  Km.;  square  kilometer,  Km2.; 
kilogram,  Kg.;  meter,  m.;  decimeter,  dm.;  centimeter,  cm.;  milli- 
meter, mm.;  square  meter,  m2.,  the  exponent  being  used  for  its 
submultiples;  cubic  meter,  m3.,  using  the  same  exponent  for  the 
submultiples. 

Momentum  is  the  force  of  motion  acquired  by  a  moving  body  by 
reason  of  the  continuance  of  its  motion.  It  is  measured  by  the 
product  of  the  mass  by  the  velocity  of  the  body. 

Monocoque  is  a  tractor  fuselage  entirely  stream-lined  with 
three-ply  veneer,  without  longerons,  struts  or  wire  bracing. 

Nacelle  is  the  short  body  of  a  pusher  aeroplane. 

Neutral  line  is  an  imaginary  line,  drawn  from  the  trailing  edge 
through  the  width  of  a  wing,  parallel  to  the  line  of  flight  when  the 
wing  has  no  lift. 

Nose  dive  is  a  nose  first  plunge  of  an  aeroplane. 

Nose  plate  is  a  specially  shaped  steel  sheet  which  connects  the 
front  ends  of  the  longerons. 

Ornithopter  is  a  machine  intended  to  fly  by  means  of  flapping 
wings. 

Outriggers  are  long  pieces  of  wood  which  support  the  tail  of  a 
pusher  machine. 

Parabola  is  a  curve  formed  by  the  intersection  of  the  surface  of 
a  cone  with  a  plane  parallel  to  one  of  its  sides. 

Parabolic  curve  is  a  curve  resembling  a  parabola. 

Plane  is  a  wooden  and  metallic -framework  covered  with  fabric. 

Plumb  line  is  a  string  with  a  weight  or  bob  attached  to  one  of 
its  ends,  used  to  determine  a  vertical  line. 

Pontoon  is  a  light,  airtight,  boat-like  float. 

Projected  propeller  surface  is  the  surface  projection  of  a  pro- 
peller into  a  plane  perpendicular  to  its  axis. 


DEFINITIONS  159 

Propeller  axis  is  the  straight  line  about  which  the  propeller 
revolves. 

Propeller  gap  is  the  distance  between  the  helicoidal  paths  of 
two  consecutive  blades. 

Propeller  pitch  is  the  distance  through  which  a  propeller  would 
advance  in  one  revolution,  if  it  moved  in  an  unyielding  medium. 
This  is  the  theoretical  pitch.  The  effective  pitch  is  the  distance 
actually  traveled  by  the  propeller  in  one  revolution.  The  pitch 
is  constant  when  the  angle  of  the  blades  varies;  it  is  varying,  when 
the  angle  is  constant. 

Propeller  pitch  angle  is  the  angle  at  which  a  propeller  blade  is 
set. 

Propeller  slip  is  the  difference  between  the  theoretical  and  ef- 
fective pitch  or  the  distance  lost  by  the  propeller  in  one  revolution. 

Propeller  thrust  is  the  force  impelled  by  the  propeller  to  the 
point  of  application. 

Propeller  torque  is  the  rotary  force  of  the  propeller.  It  produces 
an  opposite  rotary  motion  to  the  point  of  application. 

Protractor  is  an  instrument  used  for  measuring  angles. 

Pusher  machine  is  an  aeroplane  with  the  propeller  in  the  rear. 

Rib  is  a  curved  wooden  frame  used  in  a  wing  to  give  it  camber, 
carry  the  fabric  and  transfer  the  lift  from  the  fabric  to  the  spars. 
A  rib  is  composed  of  three  parts:  a  web  and  two  cap  strips.  If  the 
web  is  thick  and  solid,  the  rib  is  called  a  compression  rib;  if  the 
web  is  thin  and  lightened  by  means  of  holes  bored  in  it,  the  rib  is 
called  a  camber  rib.  There  is  also  a  false  rib,  which  is  a  strip  of 
wood  tacked  on  the  upper  front  part  of  a  wing  to  prevent  the 
fabric  from  sinking  between  the  ribs  proper. 

Rigging  or  flying  position  is  the  position  assumed  by  the  fuselage 
when  its  engine  rails  are  level  both  longitudinally  and  laterally. 

Rivet  is  a  short  bolt  without  a  thread. 

Root  end  of  a  wing  is  its  thick  end  to  which  are  attached  the 
hinges. 

Rudder  is  a  controlling  plane  hinged  vertically  to  the  rear  of  a 
machine  to  steer  it  to  the  right  or  left. 

Rudder  post  is  a  steel  tube  to  which  is  hinged  the  rudder. 

Safety  wire  is  a  fine  solid  wire  used  to  lock  a  turnbuckle;  or  to 
tie  aeroplane  wires  together  or  to  some  part  of  the  machine,  to 
avoid  their  falling  in  the  way  of  the  propeller  in  case  of  a  break. 


160  DEFINITIONS 

Screw  propeller  is  a  section  of  a  screw.  It  screws  itself  into  the 
air  and  converts  a  rotary  motion  into  a  linear  motion.  This  def- 
inition conforms  with  the  old  theory  of  the  propeller;  according  to 
the  new  theory,  a  propeller  is  a  revolving  inclined  plane. 

Serving  is  the  protective  wrapping  of  a  cord  or  a  wire  around  a 
splice. 

Sextant  is  an  instrument  for  measuring  the  angular  distance 
between  two  objects  by  means  of  a  graduated  arc,  representing 
the  sixth  part  of  a  circle,  and  a  double  reflection  from  two 
mirrors. 

Shear  stress  is  the  stress  that  tends  to  tear  a  body  in  such  a 
manner  as  to  cause  one  part  to  slide  over  the  other. 

Shielding  is  the  protection  against  the  air  resistance  offered  by  a 
body  on  another  following  in  its  wake  within  certain  limits  and 
producing  a  decrease  in  passive  drift. 

Side  slip  is  the  sideways  motion  of  an  aeroplane  toward  the 
center  of  a  turn  as  a  result  of  excessive  banking. 

Sine.    See  Trigonometry. 

.  Skid  is  a  piece  of  wood,  cane  or  tubing  used  for  supporting  or 
allowing  to  move  on  it  some  part  of  an  aeroplane,  as  the  tail,  under- 
carriage or  wing  tips. 

Skid  (to  skid)  is  to  cause  an  aeroplane  to  move  sideways 
away  from  the  center  of  a  turn  as  a  result  of  insufficient 
banking. 

Skin  friction  is  the  rubbing  of  the  air  against  the  layer  of  air 
surrounding  the  surface  of  a  moving  body. 

Span  is  the  length  of  the  main  plane  of  an  aeroplane,  measured 
from  tip  to  tip,  excluding  the  extension  when  used. 

Spar  is  a  long  piece  of  wood  within  a  wing  to  which  the  ribs  are 
attached. 

Stability  is  the  tendency  of  a  body  to  keep  its  state  of  equilibrium. 
In  an  aeroplane  there  are  three  kinds  of  stabilities:  longitudinal  or 
in  a  fore  and  aft  direction;  lateral  or  from  port  to  starboard;  di- 
rectional or  from  right  to  left. 

Stabilizing  plane  is  a  plane  that  gives  inherent  stability  to  a 
machine.  There  are  two  stabilizing  planes:  horizontal  and  vertical 
stabilizer. 

Stagger  is  the  step  disposition  of  planes,  either  forward  or  back- 
ward. 


DEFINITIONS  161 

Stagger  and  incidence  wires  are  the  internal  cross  bracing 
wires  of  the  wings. 

Stall  is  to  stop  the  forward  motion  of  an  aeroplane  through  an 
excessive  angle  of  attack. 

Straight  edge  is  a  long  piece  of  wood  or  metal  having  the  edges 
perfectly  straight,  used  to  ascertain  whether  a  surface  is  exactly 
even. 

Strain  is  the  deformation  produced  by  an  overstress. 

Stream-line  is  the  line  traced  by  the  successive  positions  of  a 
particle  of  moving  fluid.  It  is  a  continuous  curve,  as  a  fluid  can 
not  instantly  change  its  direction  of  flow  withoiit  forming  a  detri- 
mental surface  of  discontinuity. 

Stress  is  the  load  to  which  a  body  is  subjected. 

Stringer  is  a  long  strip  of  wood  running  the  full  length  of  the 
wing  through  the  web  of  the  ribs  to  prevent  them  from  rolling 
over. 

Strut  is  a  piece  of  wood  which  holds  apart  two  other  pieces  of 
wood. 

Sweepback  is  the  rearward  position  assumed  by  the  wings  when 
their  leading  edges  form  angles  with  the  lateral  axis  of  an  aero- 
plane. 

Tail  post  is  the  strut  at  the  extreme  tail  end  of  the  fuselage. 

Tail  skid  is  a  piece  of  wood  attached  under  the  tail  of  a  machine 
to  carry  the  weight  of  its  rear  portion  while  on  the  ground  and  to 
act  as  a  shock  absorber  and  brake  in  landing. 

Tail  slide  is  a  tail  first  plunge  of  an  aeroplane. 

Tension  is  the  stress  which  tends  to  elongate  a  body. 

Thrust  drift  ratio  is  the  proportion  of  the  thrust  to  the  drift  of 
a  propeller. 

Torsion  is  a  combination  of  the  compression,  tension  and  shear 
stresses. 

Tractor  machine  is  an  aeroplane  with  the  propeller  in  front. 

Trailing  edge  is  the  rear  edge  of  a  plane. 

Trigonometry  is  that  branch  of  mathematics  which  treats  of  the 
relations  of  the  sides  and  angles  of  triangles. 

In  studying  these  relations,  the  right-angled  triangle  is  taken  as 
a  base,  and  the  ratio  of  the  sides  and  hypotenuse  taken  by  two  in 
three  different  ways,  forming  six  ratios  in  all,  are  given  different 
names. 


162  DEFINITIONS 

In  any  right-angled  triangle 


92 

A  B  C,  C  being  the  right  angle,  with  reference  to  the  angle  A,  let 
B  (7  be  denoted  the  opposite  side,  and  A  C  the  adjacent  side.  Then 
we  will  have: 

opposite  side 


sine  A,  abbreviated  sin  A  = 


cosine  A 


tangent  A 
cotangent  A 


secant  A 


cosecant  A 


cos  A  = 


tan  A  = 


cot  A  = 


sec  A 


cosec  A  = 


hypotenuse 
adjacent  side 

hypotenuse 
opposite  side 
adjacent  side 
adjacent  side 
opposite  side 

hypotenuse 
adjacent  side 

hypotenuse 


opposite  side 

The  numbers  which  indicate  these  ratios  have  already  been  de- 
termined and  tabulated  for  all  angles  from  one  to  ninety  degrees. 
So  that  when  we  want  to  know  the  value  of  these  six  ratios  for  a 
given  angle,  we  have  to  look  into  the  tables  of  the  natural  functions 
of  angles.  As  in  our  calculations  we  use  mostly  sines  and  cosines, 
having  only  in  one  instance  the  use  of  tangents,  we  will  confine  our 
study  to  them  only. 

The  sum  of  the  angles  of  a  triangle  is  equal  to  two  right  angles. 
A  right  angle  is  ninety  degrees  (90°).  As  we  take  as  a  base  the 
right-angled  triangle,  it  is  evident  that,  C  being  one  right  angle, 
A  +  B  is  equal  to  one  right  angle.  Consequently,  if  A  =  1°, 
B  =  89°;  A  =  2°,  B  =  88°;  A  =  44°,  B  =  46°;  A  =  45°,  B  =  45°; 
A  =  46°,  B  =  44°;  A  =  88°,  B  =  2°;  A  =  89°,  B  =  1°.  From 


DEFINITIONS  163 

this,  we  see  that  after  we  reach  an  angle  of  45°,  the  process  is  re- 
versed; that  is,  the  sine  of  an  angle  of  1°  is  the  cosine  of  an  angle 
of  89°,  and  vice  versa.  Therefore,  instead  of  tabulating  first  all 
the  sines  of  the  angles  from  1°  to  90°  and  then  the  cosines  from  1°  to 
90°,  we  can  tabulate  the  sines  only  or,  as  it  is  commonly  done,  we 
can  write  all  the  sines  from  0°  to  45°  and  the  cosines  from  90°  to 
45°,  so  arranged  that  to  the  sine  of  an  angle  corresponds  its  cosine, 
and  vice  versa.  Where  greater  precision  is  required,  the  tables  are 
compiled  to  give  the  ratios  for  fractions  of  degrees,  that  is,  minutes, 
as  one  degree  is  sixty  minutes  (60'). 

For  our  calculations,  the  functions  of  entire  degrees  being  suffi- 
cient, the  following  table  of  sines  and  cosines  will  do: 


164 


DIFINITIONS 


NATURAL  SINES  AND  COSINES 


0 

sin 

COS 

o 

0 

sin 

COS 

0 

0 

0.00000 

1.00000 

90 

23 

0.39073 

0.92050 

67 

1 

0.01745 

0.99985 

89 

24 

0.40674 

0.91355 

66 

2 

0.03490 

0.99939 

88 

25 

0.42262 

0.90631 

65 

3 

0.05234 

0.99863 

87 

26 

0.43837 

0.89879 

64 

4 

0.06976 

0.99756 

86 

27 

0.45399 

0.89101 

63 

5 

0.08716 

0.99619 

85 

28 

0.46947 

0.88295 

62 

6 

0.10453 

0.99452 

84 

29 

0.48481 

0.87462 

61 

7 

0.12187 

0.99255 

83 

30 

0.50000 

0.86603 

60 

8 

0.13917 

0.99027 

82 

31 

0.51504 

0.85717 

59 

9 

0.15643 

0.98769 

81 

32 

0.52992 

0.84805 

58 

10 

0.17365 

0.98481 

80 

33 

0.54464 

0.83867 

57 

11 

0.19081 

0.98163 

79 

34 

0.55919 

0.82904 

56 

12 

0.20791 

0.97815 

78 

35 

0.57358 

0.81915 

55 

13 

0.22495 

0.97437 

77 

36 

0.58779 

0.80902 

54 

14 

0.24192 

0.97030 

76 

37 

0.60182 

0.79864 

53 

15 

0.25882 

0.96593 

75 

38 

0.61566 

0.78801 

52 

16 

0.27564 

0.96126 

74 

39 

0.62932 

0.77715 

51 

17 

0.29237 

0.95630 

73 

40 

0.64279 

0.76604 

50 

18 

0.30902 

0.95106 

72 

41 

0.65606 

0.75471 

49 

19 

0.32557 

0.94552 

71 

42 

0.66913 

0.74314 

48 

20 

0.34202 

0.93969 

70 

43 

0.68200 

0.73135 

47 

21 

0.35837 

0.93358 

69 

44 

0.69466 

0.71934 

46 

22 

0.37461 

0.92718 

68 

45 

0.70711 

0.70711 

45 

cos 

sin 

cos 

sin 

DEFINITIONS  165 

This  is  the  table  of  the  natural  sines  and  cosines  of  angles,  to 
be  distinct  from  the  logarithmic  functions,  which  do  not  enter  in 
our  calculations. 

If  we  want  to  know  the  sine  and  cosine  of  an  angle  of  10°,  for 
instance,  we  look  in  the  table  for  the  angle  of  10°  and  we  find:  sin 
10°  =  0.17365,  cos  10°  =  0.98481.  If,  instead,  we  look  for  the 
sine  and  cosine  of  an  angle  of  80°,  we  find:  sin  80°  =  0.98481,  cos 
80°  =  0.17365.  In  this  way,  we  will  be  able  to  find  the  sine  and 
cosine  of  any  angle. 

If,  as  a  result  of  our  calculations,  we  find  the  sine  of  an  angle  and 
we  want  to  know  its  value  in  degrees,  we  look  for  this  sine  in  the 
tables  and  see  what  degree  corresponds  to  it;  but  if  we  do  not  find 
a  sine  exactly  equal  to  the  one  we  are  looking  for,  it  means  that  the 
angle  is  not  expressed  by  an  entire  number  of  degrees,  and  we 
have  to  look  for  it  in  more  detailed  tables. 

Suppose,  for  instance,  that  we  want  to  know  what  angle  in 
degrees  corresponds  to  the  sine  0.105.  In  the  table,  we  find  that 
the  nearest  approach  to  it  is  sin  6°  =  0.10453;  therefore,  the  degree 
of  our  angle  is  between  6°  and  7°;  but,  evidently,  much  nearer  to 
6°  than  it  is  to  7°.  And  if  we  look  for  it  in  a  more  detailed  table, 
we  find  our  angle  expressed  in  degrees  and  factions  of  a  degree  or 
minutes. 

Turnbuckle  is  a  coupling  with  a  barrel  and  a  right  hand  and  a 
left  hand  eye  screw  or  shank  used  to  regulate  the  length  and  tension 
of  wires.  The  right  hand  screw  shank,  which  sometimes  is  split  or 
forked,  is  always  attached  to  a  fitting,  while  the  left  hand  screw 
shank  is  attached  to  the  wire.  This  is  done  to  determine  the  turn- 
ing direction  of  the  barrel  in  tightening  or  loosening  a  wire,  as,  in 
this  case,  the  operation  is  that  of  an  ordinary  right  hand  screw  nut. 
The  come  and  go  is  the  distance  the  shanks  can  be  screwed  in  or  out. 

Undercarriage  is  that  part  of  an  aeroplane  designed  to  support 
it  when  at  rest,  to  absorb  the  shock  of  landing  and  to  give  clearance 
to  the  propeller  and  wings. 

Veneer  is  a  thin  layer  of  wood. 

Vertical  stabilizer  or  fin  is  a  triangular  plane  bolted  at  the  upper 
part  of  the  horizontal  stabilizer  to  give  inherent  directional  stability 
to  an  aeroplane. 

Volplane  is  a  glide. 

Wash  in  is  an  increase  in  the  angle  of  incidence. 


166  DEFINITIONS 

Wash  out  is  a  decrease  in  the  angle  of  incidence. 

Web  is  the  central  part  of  a  rib. 

Wind  screen  is  a  shield  placed  in  front  of  the  cockpit  to  protect 
the  aviator  from  the  effect  of  the  wind. 

Wind  tunnel  is  a  tube  through  which  air  is  forced  or  drawn  by 
means  of  rotating  fans. 

Wing  is  a  fabric  covered,  cambered,  wooden  and  metallic  frame. 

Wing  tip  is  the  extreme  thin  end  of  a  wing  opposite  the  root  end. 


INDEX 


Acetone,  147 

Ackerman  wheel,  40 

Active  drift,  14,  154 

Aeroplane  14,  147 

Aerotelephone,  118 

Aileron,  44,  70,  147 

Aileron  control,  50,  53 

Aileron  droop,  78 

Air  resistance,  1,  3, 14, 16, 134, 135 

Air  speed  meter,  147 

Air  spill,  12,  139 

Algebra,  147 

Altimeter,  151 

Aluminum,  62 

Anemometer,  151 

Angle  of  attack,  151 

Angle  of  incidence,  3,  9,  10,  77,  135 

Angle  of  sweepback,  28,  151 

Antidrift  wire,  42,  43,  151 

Ascending,  122 

Ash,  61 

Aspect  ratio,  11,  151 

Assembling,  70 

Assembling  order,  34 

Aviation,  1,  151 

Axle,  38,  101 

B 

Balance  wire,  50 
Banana  oil,  68,  152 
Bank,  152 
Bank  indicator,  156 
Barograph,  152 
Barometer,  152 
Baseball  stitch,  113 


Battens,  55,  56 

Bay,  152 

Bending,  58,  81,  152 

Biplane,  14 

Bird's  muscular  power,  5 

Blow  torch,  107,  152 

Bolts,  66,  71 

Bottom  rudder,  123 

Bracing  wires,  42,  47,  72 

Brake,  41 

Brushes,  69 

Bulkheads,  54,  55 


Cabane,  45,  46,  152 

Camber,  152 

Camber  ribs,  44,  159 

Cambered  planes,  4,  5,  10,  138 

Cap  strip,  44,  152 

Castellated  nut,  66,  152 

Cavitation,  90,  152 

Cedar,  61 

Cellulose,  152 

Cellulose  nitrate,  68 

Center  line  of  pressure,  152 

Center  of  gravity,  18,  19,  20,  21, 
22,  29,  42,  152 

Center  of  lift,  18,  29,  152 
Center  of  pressure,  10,  21,  22,  136, 

152 

Center  of  resistance,  18,  29,  153 
Center  of  thrust,  18,  29,  153 
Center  section,  42,  70,  75,  153 
Center  section  panel,  42 
Centrifugal  force,  20,  153 
Centripetal  force,  153 


167 


168 


INDEX 


Chord,  11,  153 

Circular  flight,  20 

Cleaning,  100 

Clevis  pin,  66,  153 

Cockpit,  36,  153 

Coefficient  of  curvature,  139 

Coefficient  of  resistance,  134 

Compass,  153 

Compensating  wire,  53 

Compression,  58,  153 

Compression  rib,  44 

Control  cable,  63,  101 

Control  column,  50 

Control  lever,  153 

Control  wire,  50,  72 

Controlling  planes,  81,  82,  153 

Controls,  49,  80,  102,  121,  123 

Cord  stay,  63 

Cotter  pin,  66,  71,  153 

Cotton,  68 

Cowl,  37 

Cowling,  36 

Cross  bracing  wires,  34,  36,  38,  42, 

43,  73 
Curtiss  boss,  98 


Decalage,  24,  153 
Degree  of  curvature,  8 
Directional  stability,  22 
Disk  wheels,  39,  154 
Dope,  68,  113,  154 
Doping,  69 
Dowels,  44,  154 
Drain  holes,  54,  55 
Drift,  2,  5,  7,  136,  138,  140,  154 
Drift  meter,  154 
Drift  wires,  42,  43,  46,  154 
Droop,  154 
Drum,  50 
Dual  control,  53 

Dual  control  system  of  instruc- 
tion, 117,  118 


E 

Eddy,  154 

Effective  area,  11,  139 
Efficiency  of  construction,  154 
Elementary  flying,  120 
Elevators,  45,  79,  80,  123,  154 
Elevators  control,  51,  53,  72 
Elevators  droop,  80 
Empennage,  44,  72,  78,  155 
Engine  rails,  34,  35,  61,  82 
Engine  rails  support,  34,  72 
Engine  section,  36,  37 
Equation,  147 
Equilibrium,  17,  155 
Extension,  14,  155 


Fabric,  67,  101,  113 

Factor  of  safety,  59,  60,  155 

Fairing,  36,  155 

Fair  lead,  66,  155 

False  ribs,  43,  44,  159 

Ferrule,  64,  155 

Ferrule  and  loop,  65,  104 

Figure  8  turns,  122 

Fineness,  15,  155 

Fittings,  34,  35,  63,  101,  103,  155 

Flat  planes,  1 

Flight  hints,  116 

Flutter,  97 

Flux,  105,  106,  155 

Flying  angle  of  incidence,  10,  151 

Flying  boat,  56,  155 

Flying  wires,  45,  47,  71,  155 

Foot  rudder  bar,  50,  155 

Forced  landing,  102 

Formula,  147 

Formula  of  A  H  P,  140,  141 

Formula  of  center  of  pressure,  136 

Formula  of  drift,  137 

Formula  of  E  H  P,  139 

Formula  of  H  P,  138,  139 


INDEX 


169 


Formula  of  lift,  137,  142 
Formula  of  passive  drift,  140 
Formula  of  sin  a  cos  a,  143,  144 
Formula  of  S,  143 
Formula  of  T  H  P,  141 
Formula  of  V,  143 
Formula  of  W,  143 
Formulae  in  British  Units,  145 
Furnace,  110 
Fuselage,  34,  37,  70,  72,  155 

G 

Gap,  12,  88,  156 
Glider,  8 
Gliding,  29,  122 
Gliding  angle,  29,  151 
Gliding  path,  30 
Gnome  boss,  98 

H 

Hand  holes,  54,  55,  81 

Hangar,  156 

Helicopter,  8,  141,  156 

High  aspect  ratio,  12 

Higher  wing,  125 

Hinges,  43 

Holes  in  fabric,  114 

Holes  in  wood,  103 

Horizontal  component,  2 

Horizontal  equivalent,  7,  27,  156 

Horizontal  projection,  7 

Horizontal  stabilizer,  24,  44,  45, 
72,  78,  156 

Hydraulic  pneumatic  shock  ab- 
sorbers, 40 

Hydroaeroplane,  54,  56,  156 

Hydrochloric  acid,  106,  156 

Hydroplane,  156 


Inclined  plane,  7,  135 
Inclinometer,  156 


Inertia,  156 
Inherent  stability,  23 
Inspection,  100 
Inspection  cover,  156 
Interference,  12,  88,  156 
Interplane  struts,  47 
Irish  linen,  67 

K 

Keel  surface,  57,  156 
King  post,  47,  156 
Kite,  1 


Landing,  122 

Landing  wires,  45,  47,  71,  156 

Lateral  dihedral  angle,  26,  151 

Lateral  stability,  19 

Laterometer,  156 

Leading  edge,  5,  9,  43,  61,  70,  75, 

76,  157 

Left  side  of  aeroplane,  72 
Level,  81,  157 
Lift,  2,  5,  7,  11,  12,  13,  136,  137, 

140,  157 

Lift  drift  ratio,  7,  157 
Lifting  tail,  24 
Line,  81 

Line  of  flight,  9,  157 
Locking  arrangement,  101 
Locking  wires,  64 
Longerons,  34,  61,  157 
Longitudinal  dihedral  angle,  24, 

76,  151 

Longitudinal  stability,  18 
Loop,  65,  112,  129,  157 
Low  aspect  ratio,  12 
Lower  wing,  125 

M 

Mahogany,  61 
Maintenance,  100 


170 


INDEX 


Margin  of  lift,  157 

Margin  of  power,  157 

Materials  of  construction,  57 

Maxim  angle  of  incidence,  151 

Metal,  62 

Metallic  sheets,  67 

Methods  of  instruction,  flying,  116 

Metric  system,  157 

Minimum  angle  of  incidence,  151 

Momentum,  158 

Monel  metal,  62 

Monocoque,  37,  158 

Monoplane,  14 

Moving  parts  of  aeroplane,  101 

Multiplane,  14 

N 

Nacelle,  38,  158 

Negative  angle  of  incidence,  10 

Negative  stagger,  14 

Neutral  line,  9,  158 

Neutral  position  of  controls,  51 

Non-lifting  tail,  26 

Non-skid  fin,  57 

Nose  dive,  158 

Nose  plate,  34,  36,  158 

Nuts,  66,  81 

O 

Oleo  pneumatic  shock  absorbers, 
40 

One-seat  machine  method  of  in- 
struction, flying,  117 

Optimum  angle  of  incidence,  151 

Ornithopter,  5,  141,  142,  158 

Outriggers,  49,  158 

Overhang,  76 


Paint,  103 

Parabola,  8,  158 

Parabolic  curve,  8,  158 

Passive  drift,  14,  16,  140,  145,  154 


Patching,  113,  114 

Phillip's  coefficient,  16 

Picketing,  102 

Pins,  71 

Plane,  158 

Planing  fins,  55 

Plumb  line,  74,  158 

Plywood,  60 

Pontoons,  54,  56,  57,  158 

Positive  angle  of  incidence,  10 

Positive  stagger,  14 

Power  plant,  48 

Pressed  steel,  103 . 

Propeller,  14,  72,  83,  102,  141',  160 

Propeller  alignment,  99 

Propeller  angle  of  pitch,  84,  87,  92, 
96,  159 

Propeller  apparent  slip,  91 

Propeller  axis,  159 

Propeller  balance,  95 

Propeller  balance  error,  96 

Propeller  blade  joints,  97 

Propeller  blade  length,  97 
Propeller  blade  straightness,  97 
Propeller  blade  warpage,  96 
Propeller  blade  width,  97 
Propeller  bolt  holes,  97 
Propeller  boss,  97 
Propeller  developed  circumference 

of  revolution,  86 
Propeller  diameter,  89 
Propeller  effective  pitch,  84,  89 
Propeller  efficiency,  139 
Propeller  gap,  159 
Propeller  hub,  85 
Propeller  hub  holes,  97 
Propeller  manufacture,  93 
Propeller,  metallic,  94 
Propeller  metallic  tips,  95 
Propeller  mounting,  99 
Propeller  negative  slip,  91 
Propeller  pitch,  84,  93,  159 
Propeller  pitch  line,  86,  87 


INDEX 


171 


Propeller  pitch  ratio,  89 
Propeller  position,  91 
Propeller  problems,  92 
Propeller  projected  surface,  158 
Propeller  slip,  84,  159 
Propeller  test,  96 
Propeller  thrust,  159 
Propeller  thrust  drift  ratio, .89,  161 
Propeller  torque,  31,  79,  90,  159 
Propeller  triangle  of  pitch,  86 
Propeller,  wooden,  94 
Protractor,  159 
Punctures  in  tires,  115 
Pusher  machine,  48,  49,  159 

R 

Radius  of  glide,  30 

Radius  rod,  38,  41 

Reenforcing  struts,  34,  55 

Repairs,  fabric,  102 

Repairs,  metal,  103 

Repairs,  rubber,   115 

Repairs,  wood,  102 

Ribs,  43,  61,  159 

Rigger's  angle  of  incidence,  10 

Rigging,  70 

Rigging  care  and  faults,  80 

Rigging  position,  70,  72,  159 

Right  side  of  aeroplane,  72 

Rivet,  66,  159 

Roll,  131 

Root  end,  68,  159 

Rubber  shock  absorbers,  39,  101 

Rudder,  22,  44,  123,  159 

Rudder  control,  51,  53,  72,  79 

Rudder  post,  34,  36,  101,  159 


Safety  wire,  159 
Sagging,  21 
Screws,  67 
Sea  Island  cotton,  68 


Self-instruction,  flying,  116 

Semilifting  tail,  25 

Serving,  66,  105,  160 

Sextant,  160 

Sharp  turns,  122 

Shear  stress,  58,  160 

Shielding,  16,  160 

Shock  absorber  fittings,  38 

Side  slip,  125,  160 

Silk,  68 

Sine,  160 

Skid,  160 

Skin  friction,  16,  134,  160 

Slight  climbing  turns,  122 

Slight  climbs,  122 

Slight    deviations    from    straight 

flight,  121 

Slight  sideways  motions,  122 
Soap,  100 
Solder,  105 
Soldering,  105 
Soldering  iron,  111 
Solid  wire,  63 
Solo  flying,  122 
Span,  11,  160 
Spar  varnish,  69 
Spars,  43,  61,  160 
Spin,  126 
Spiraling,  122 
Spliced  loop,  66 
Spokes,  101 
Spreader,  38,  39 
Spruce,  61 
Stability,  17,  160 
Stability  plane,  160 
Stagger,  14,  75,  78,  160 
Stagger  and  incidence  wires,  47, 

75,  161 
Stall,  161 
Steel,  62 

Steel  tubing,  62,  63 
Steeper  climbs  and  descents,  122 
Step,  54,  55 


172 


INDEX 


Stick,  52 

Stick  control,  52,  54,  119 

Straight  edge,  81,  161 

Straight  flight,  19,  121 

Strain,  58,  161 

Strand  stay,  63 

Stranded  wire,  63 

Stream-line,  4,  15,  161 

Strength  of  materials,  58 

Stress,  58,  161 

Stringers,  43,  44,  161 

Strut,  13,  34,  35,  38,  42,  61,  70,  71, 

73,  101,  161 
Stunts,  122 
Superposed  planes,  12 
Sweepback,  28,  161 


Table  of  sines  and  cosines,  164 

Tacks,  67 

Tail,  24 

Tail  post,  34,  35,  161 

Tail  section,  36 

Tail  skid,  34,  36, 161 

Tail  skid  post,  101 

Tail  slide,  129,  161 

Tandem  planes,  3,  4 

Taxying,  119 

Tears  in  fabric,  113 

Tension,  58,  161 

Thimble,  65 

Thimble  and  loop  wrapped  and 

soldered,  65,  105 
Three-ply  veneer,  43 
Tires,  39 
Tools,  81 
Top  rudder,  123 
Torsion,  58,  161 
Total  drift,  154 
Tractor  aeroplane,  48,  161 
Trailing  edge,  9,  43,  61,  71,  75,  81, 

161 
Trigonometry,  161 

Printed  in  the  United 


Triplane,  14 

Truing,  72 

Turnbuckle,  34,  36,  63,  81,  165 

Turtleback,  37 

Twin-motor  machine,  48 

U 

Undercarriage,  38,  41,  70,  74,  165 


Veneer,  43,  60,  165 

Vent  pipe,  54,  55 

Vertical  bank,  123 

Vertical  component,  2 

Vertical  equivalent,  7 

Vertical  projection,  7,  79 

Vertical  stabilizer,  22,  28,  44,  45, 

72,  78,  165 
Volplane,  165 

W 

Walnut,  61 

Wash  in,  31,  78,  165 

Wash  out,  31,  78,  166 

Washers,  66,  103 

Web,  44,  166 

Wheel  control,  50,  54,  119 

Wheels,  38,  39 

White  pine,  61 

Wind  screen,  166 

Wind  tunnel,  166 

Wing  covering,  68 

Wing  tip,  43,  166 

Wings,  43,  70,  75 

Wire  test,  103 

Wires,  45,  63,  73,  76,  81,  101 

Wood,  60 

Wrapped  and  soldered  loop,  65, 105 

Wrenches,  81 


Zero  angle  of  incidence,  3 
Zero  stagger,  14 
States  of  America 


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